mirror of
https://git.postgresql.org/git/postgresql.git
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character in identifiers. The first change eliminates the current need to put spaces around parameter references, as in "x<=$2". The second change improves compatibility with Oracle and some other RDBMSes. This was discussed and agreed to back in January, but did not get done.
1291 lines
44 KiB
Plaintext
1291 lines
44 KiB
Plaintext
<!--
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$Header: /cvsroot/pgsql/doc/src/sgml/syntax.sgml,v 1.79 2003/06/19 23:22:40 tgl Exp $
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-->
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<chapter id="sql-syntax">
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<title>SQL Syntax</title>
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<indexterm zone="sql-syntax">
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<primary>syntax</primary>
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<secondary>SQL</secondary>
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</indexterm>
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<para>
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This chapter describes the syntax of SQL. It forms the foundation
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for understanding the following chapters which will go into detail
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about how the SQL commands are applied to define and modify data.
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</para>
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<para>
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We also advise users who are already familiar with SQL to read this
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chapter carefully because there are several rules and concepts that
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are implemented inconsistently among SQL databases or that are
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specific to <productname>PostgreSQL</productname>.
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</para>
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<sect1 id="sql-syntax-lexical">
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<title>Lexical Structure</title>
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<para>
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SQL input consists of a sequence of
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<firstterm>commands</firstterm>. A command is composed of a
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sequence of <firstterm>tokens</firstterm>, terminated by a
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semicolon (<quote>;</quote>). The end of the input stream also
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terminates a command. Which tokens are valid depends on the syntax
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of the particular command.
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</para>
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<para>
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A token can be a <firstterm>key word</firstterm>, an
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<firstterm>identifier</firstterm>, a <firstterm>quoted
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identifier</firstterm>, a <firstterm>literal</firstterm> (or
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constant), or a special character symbol. Tokens are normally
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separated by whitespace (space, tab, newline), but need not be if
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there is no ambiguity (which is generally only the case if a
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special character is adjacent to some other token type).
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</para>
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<para>
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Additionally, <firstterm>comments</firstterm> can occur in SQL
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input. They are not tokens, they are effectively equivalent to
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whitespace.
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</para>
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<para>
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For example, the following is (syntactically) valid SQL input:
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<programlisting>
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SELECT * FROM MY_TABLE;
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UPDATE MY_TABLE SET A = 5;
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INSERT INTO MY_TABLE VALUES (3, 'hi there');
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</programlisting>
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This is a sequence of three commands, one per line (although this
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is not required; more than one command can be on a line, and
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commands can usefully be split across lines).
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</para>
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<para>
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The SQL syntax is not very consistent regarding what tokens
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identify commands and which are operands or parameters. The first
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few tokens are generally the command name, so in the above example
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we would usually speak of a <quote>SELECT</quote>, an
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<quote>UPDATE</quote>, and an <quote>INSERT</quote> command. But
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for instance the <command>UPDATE</command> command always requires
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a <token>SET</token> token to appear in a certain position, and
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this particular variation of <command>INSERT</command> also
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requires a <token>VALUES</token> in order to be complete. The
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precise syntax rules for each command are described in <xref linkend="reference">.
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</para>
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<sect2 id="sql-syntax-identifiers">
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<title>Identifiers and Key Words</title>
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<indexterm zone="sql-syntax-identifiers">
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<primary>identifiers</primary>
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</indexterm>
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<indexterm zone="sql-syntax-identifiers">
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<primary>key words</primary>
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<secondary>syntax</secondary>
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</indexterm>
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<para>
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Tokens such as <token>SELECT</token>, <token>UPDATE</token>, or
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<token>VALUES</token> in the example above are examples of
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<firstterm>key words</firstterm>, that is, words that have a fixed
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meaning in the SQL language. The tokens <token>MY_TABLE</token>
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and <token>A</token> are examples of
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<firstterm>identifiers</firstterm>. They identify names of
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tables, columns, or other database objects, depending on the
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command they are used in. Therefore they are sometimes simply
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called <quote>names</quote>. Key words and identifiers have the
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same lexical structure, meaning that one cannot know whether a
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token is an identifier or a key word without knowing the language.
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A complete list of key words can be found in <xref
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linkend="sql-keywords-appendix">.
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</para>
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<para>
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SQL identifiers and key words must begin with a letter
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(<literal>a</literal>-<literal>z</literal>, but also letters with
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diacritical marks and non-Latin letters) or an underscore
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(<literal>_</literal>). Subsequent characters in an identifier or
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key word can be letters, underscores, digits
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(<literal>0</literal>-<literal>9</literal>), or dollar signs
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(<literal>$</>). Note that dollar signs are not allowed in identifiers
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according to the letter of the SQL standard, so their use may render
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applications less portable.
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The SQL standard will not define a key word that contains
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digits or starts or ends with an underscore, so identifiers of this
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form are safe against possible conflict with future extensions of the
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standard.
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</para>
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<para>
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The system uses no more than <symbol>NAMEDATALEN</symbol>-1
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characters of an identifier; longer names can be written in
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commands, but they will be truncated. By default,
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<symbol>NAMEDATALEN</symbol> is 64 so the maximum identifier
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length is 63. If this limit is problematic, it can be raised by
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changing the <symbol>NAMEDATALEN</symbol> constant in
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<filename>src/include/postgres_ext.h</filename>.
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</para>
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<para>
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<indexterm>
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<primary>case sensitivity</primary>
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<secondary>SQL commands</secondary>
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</indexterm>
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Identifier and key word names are case insensitive. Therefore
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<programlisting>
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UPDATE MY_TABLE SET A = 5;
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</programlisting>
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can equivalently be written as
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<programlisting>
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uPDaTE my_TabLE SeT a = 5;
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</programlisting>
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A convention often used is to write key words in upper
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case and names in lower case, e.g.,
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<programlisting>
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UPDATE my_table SET a = 5;
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</programlisting>
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</para>
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<para>
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<indexterm>
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<primary>quotes</primary>
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<secondary>and identifiers</secondary>
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</indexterm>
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There is a second kind of identifier: the <firstterm>delimited
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identifier</firstterm> or <firstterm>quoted
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identifier</firstterm>. It is formed by enclosing an arbitrary
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sequence of characters in double-quotes
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(<literal>"</literal>). <!-- " font-lock mania --> A delimited
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identifier is always an identifier, never a key word. So
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<literal>"select"</literal> could be used to refer to a column or
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table named <quote>select</quote>, whereas an unquoted
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<literal>select</literal> would be taken as a key word and
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would therefore provoke a parse error when used where a table or
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column name is expected. The example can be written with quoted
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identifiers like this:
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<programlisting>
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UPDATE "my_table" SET "a" = 5;
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</programlisting>
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</para>
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<para>
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Quoted identifiers can contain any character other than a double
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quote itself. To include a double quote, write two double quotes.
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This allows constructing table or column names that would
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otherwise not be possible, such as ones containing spaces or
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ampersands. The length limitation still applies.
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</para>
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<para>
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Quoting an identifier also makes it case-sensitive, whereas
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unquoted names are always folded to lower case. For example, the
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identifiers <literal>FOO</literal>, <literal>foo</literal>, and
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<literal>"foo"</literal> are considered the same by
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<productname>PostgreSQL</productname>, but <literal>"Foo"</literal>
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and <literal>"FOO"</literal> are different from these three and
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each other.
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<footnote>
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<para>
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The folding of unquoted names to lower case in <productname>PostgreSQL</>
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is incompatible with the SQL standard, which says that unquoted
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names should be folded to upper case. Thus, <literal>foo</literal>
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should be equivalent to <literal>"FOO"</literal> not
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<literal>"foo"</literal> according to the standard. If you want to
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write portable applications you are advised to always quote a particular
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name or never quote it.
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</para>
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</footnote>
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</para>
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</sect2>
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<sect2 id="sql-syntax-constants">
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<title>Constants</title>
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<indexterm zone="sql-syntax-constants">
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<primary>constants</primary>
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</indexterm>
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<para>
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There are three kinds of <firstterm>implicitly-typed
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constants</firstterm> in <productname>PostgreSQL</productname>:
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strings, bit strings, and numbers.
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Constants can also be specified with explicit types, which can
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enable more accurate representation and more efficient handling by
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the system. The implicit constants are described below; explicit
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constants are discussed afterwards.
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</para>
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<sect3 id="sql-syntax-strings">
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<title>String Constants</title>
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<indexterm zone="sql-syntax-strings">
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<primary>character strings</primary>
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<secondary>constants</secondary>
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</indexterm>
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<para>
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<indexterm>
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<primary>quotes</primary>
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<secondary>escaping</secondary>
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</indexterm>
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A string constant in SQL is an arbitrary sequence of characters
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bounded by single quotes (<quote>'</quote>), e.g., <literal>'This
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is a string'</literal>. SQL allows single quotes to be embedded
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in strings by typing two adjacent single quotes (e.g.,
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<literal>'Dianne''s horse'</literal>). In
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<productname>PostgreSQL</productname> single quotes may
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alternatively be escaped with a backslash (<quote>\</quote>,
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e.g., <literal>'Dianne\'s horse'</literal>).
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</para>
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<para>
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C-style backslash escapes are also available:
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<literal>\b</literal> is a backspace, <literal>\f</literal> is a
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form feed, <literal>\n</literal> is a newline,
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<literal>\r</literal> is a carriage return, <literal>\t</literal>
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is a tab, and <literal>\<replaceable>xxx</replaceable></literal>,
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where <replaceable>xxx</replaceable> is an octal number, is the
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character with the corresponding ASCII code. Any other character
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following a backslash is taken literally. Thus, to include a
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backslash in a string constant, type two backslashes.
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</para>
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<para>
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The character with the code zero cannot be in a string constant.
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</para>
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<para>
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Two string constants that are only separated by whitespace
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<emphasis>with at least one newline</emphasis> are concatenated
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and effectively treated as if the string had been written in one
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constant. For example:
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<programlisting>
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SELECT 'foo'
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'bar';
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</programlisting>
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is equivalent to
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<programlisting>
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SELECT 'foobar';
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</programlisting>
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but
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<programlisting>
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SELECT 'foo' 'bar';
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</programlisting>
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is not valid syntax. (This slightly bizarre behavior is specified
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by <acronym>SQL</acronym>; <productname>PostgreSQL</productname> is
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following the standard.)
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</para>
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</sect3>
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<sect3 id="sql-syntax-bit-strings">
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<title>Bit-String Constants</title>
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<indexterm zone="sql-syntax-bit-strings">
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<primary>bit strings</primary>
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<secondary>constants</secondary>
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</indexterm>
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<para>
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Bit-string constants look like string constants with a
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<literal>B</literal> (upper or lower case) immediately before the
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opening quote (no intervening whitespace), e.g.,
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<literal>B'1001'</literal>. The only characters allowed within
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bit-string constants are <literal>0</literal> and
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<literal>1</literal>.
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</para>
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<para>
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Alternatively, bit-string constants can be specified in hexadecimal
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notation, using a leading <literal>X</literal> (upper or lower case),
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e.g., <literal>X'1FF'</literal>. This notation is equivalent to
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a bit-string constant with four binary digits for each hexadecimal digit.
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</para>
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<para>
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Both forms of bit-string constant can be continued
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across lines in the same way as regular string constants.
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</para>
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</sect3>
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<sect3>
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<title>Numeric Constants</title>
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<indexterm>
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<primary>numeric</primary>
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<secondary>constants</secondary>
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</indexterm>
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<para>
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Numeric constants are accepted in these general forms:
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<synopsis>
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<replaceable>digits</replaceable>
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<replaceable>digits</replaceable>.<optional><replaceable>digits</replaceable></optional><optional>e<optional>+-</optional><replaceable>digits</replaceable></optional>
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<optional><replaceable>digits</replaceable></optional>.<replaceable>digits</replaceable><optional>e<optional>+-</optional><replaceable>digits</replaceable></optional>
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<replaceable>digits</replaceable>e<optional>+-</optional><replaceable>digits</replaceable>
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</synopsis>
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where <replaceable>digits</replaceable> is one or more decimal
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digits (0 through 9). At least one digit must be before or after the
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decimal point, if one is used. At least one digit must follow the
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exponent marker (<literal>e</literal>), if one is present.
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There may not be any spaces or other characters embedded in the
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constant. Note that any leading plus or minus sign is not actually
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considered part of the constant; it is an operator applied to the
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constant.
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</para>
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<para>
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These are some examples of valid numeric constants:
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<literallayout>
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42
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3.5
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4.
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.001
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5e2
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1.925e-3
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</literallayout>
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</para>
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<para>
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A numeric constant that contains neither a decimal point nor an
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exponent is initially presumed to be type <type>integer</> if its
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value fits in type <type>integer</> (32 bits); otherwise it is
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presumed to be type <type>bigint</> if its
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value fits in type <type>bigint</> (64 bits); otherwise it is
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taken to be type <type>numeric</>. Constants that contain decimal
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points and/or exponents are always initially presumed to be type
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<type>numeric</>.
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</para>
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<para>
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The initially assigned data type of a numeric constant is just a
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starting point for the type resolution algorithms. In most
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cases the constant will be automatically coerced to the most
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appropriate type depending on context. When necessary, you
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can force a numeric value to be interpreted as a specific
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data type by casting it. For example, you can force a numeric
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value to be treated as type <type>real</> (<type>float4</>)
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by writing
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<programlisting>
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REAL '1.23' -- string style
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1.23::REAL -- PostgreSQL (historical) style
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</programlisting>
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</para>
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</sect3>
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<sect3 id="sql-syntax-constants-generic">
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<title>Constants of Other Types</title>
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<indexterm>
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<primary>data types</primary>
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<secondary>constants</secondary>
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</indexterm>
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<para>
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A constant of an <emphasis>arbitrary</emphasis> type can be
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entered using any one of the following notations:
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<synopsis>
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<replaceable>type</replaceable> '<replaceable>string</replaceable>'
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'<replaceable>string</replaceable>'::<replaceable>type</replaceable>
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CAST ( '<replaceable>string</replaceable>' AS <replaceable>type</replaceable> )
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</synopsis>
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The string's text is passed to the input conversion
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routine for the type called <replaceable>type</replaceable>. The
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result is a constant of the indicated type. The explicit type
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cast may be omitted if there is no ambiguity as to the type the
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constant must be (for example, when it is passed as an argument
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to a non-overloaded function), in which case it is automatically
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coerced.
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</para>
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<para>
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It is also possible to specify a type coercion using a function-like
|
|
syntax:
|
|
<synopsis>
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<replaceable>typename</replaceable> ( '<replaceable>string</replaceable>' )
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</synopsis>
|
|
but not all type names may be used in this way; see <xref
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linkend="sql-syntax-type-casts"> for details.
|
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</para>
|
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<para>
|
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The <literal>::</literal>, <literal>CAST()</literal>, and
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|
function-call syntaxes can also be used to specify run-time type
|
|
conversions of arbitrary expressions, as discussed in <xref
|
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linkend="sql-syntax-type-casts">. But the form
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<literal><replaceable>type</replaceable> '<replaceable>string</replaceable>'</literal>
|
|
can only be used to specify the type of a literal constant.
|
|
Another restriction on
|
|
<literal><replaceable>type</replaceable> '<replaceable>string</replaceable>'</literal>
|
|
is that it does not work for array types; use <literal>::</literal>
|
|
or <literal>CAST()</literal> to specify the type of an array constant.
|
|
</para>
|
|
</sect3>
|
|
|
|
<sect3>
|
|
<title>Array constants</title>
|
|
|
|
<indexterm>
|
|
<primary>arrays</primary>
|
|
<secondary>constants</secondary>
|
|
</indexterm>
|
|
|
|
<para>
|
|
The general format of an array constant is the following:
|
|
<synopsis>
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'{ <replaceable>val1</replaceable> <replaceable>delim</replaceable> <replaceable>val2</replaceable> <replaceable>delim</replaceable> ... }'
|
|
</synopsis>
|
|
where <replaceable>delim</replaceable> is the delimiter character
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|
for the type, as recorded in its <literal>pg_type</literal>
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|
entry. (For all built-in types, this is the comma character
|
|
<quote><literal>,</literal></>.) Each <replaceable>val</replaceable> is either a constant
|
|
of the array element type, or a subarray. An example of an
|
|
array constant is
|
|
<programlisting>
|
|
'{{1,2,3},{4,5,6},{7,8,9}}'
|
|
</programlisting>
|
|
This constant is a two-dimensional, 3-by-3 array consisting of three
|
|
subarrays of integers.
|
|
</para>
|
|
|
|
<para>
|
|
Individual array elements can be placed between double-quote
|
|
marks (<literal>"</literal>) <!-- " --> to avoid ambiguity
|
|
problems with respect to whitespace. Without quote marks, the
|
|
array-value parser will skip leading whitespace.
|
|
</para>
|
|
|
|
<para>
|
|
(Array constants are actually only a special case of the generic
|
|
type constants discussed in the previous section. The constant
|
|
is initially treated as a string and passed to the array input
|
|
conversion routine. An explicit type specification might be
|
|
necessary.)
|
|
</para>
|
|
</sect3>
|
|
</sect2>
|
|
|
|
|
|
<sect2 id="sql-syntax-operators">
|
|
<title>Operators</title>
|
|
|
|
<indexterm zone="sql-syntax-operators">
|
|
<primary>operators</primary>
|
|
<secondary>syntax</secondary>
|
|
</indexterm>
|
|
|
|
<para>
|
|
An operator is a sequence of up to <symbol>NAMEDATALEN</symbol>-1
|
|
(63 by default) characters from the following list:
|
|
<literallayout>
|
|
+ - * / < > = ~ ! @ # % ^ & | ` ?
|
|
</literallayout>
|
|
|
|
There are a few restrictions on operator names, however:
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
<literal>--</literal> and <literal>/*</literal> cannot appear
|
|
anywhere in an operator name, since they will be taken as the
|
|
start of a comment.
|
|
</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>
|
|
A multiple-character operator name cannot end in <literal>+</> or <literal>-</>,
|
|
unless the name also contains at least one of these characters:
|
|
<literallayout>
|
|
~ ! @ # % ^ & | ` ?
|
|
</literallayout>
|
|
For example, <literal>@-</literal> is an allowed operator name,
|
|
but <literal>*-</literal> is not. This restriction allows
|
|
<productname>PostgreSQL</productname> to parse SQL-compliant
|
|
queries without requiring spaces between tokens.
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
|
|
<para>
|
|
When working with non-SQL-standard operator names, you will usually
|
|
need to separate adjacent operators with spaces to avoid ambiguity.
|
|
For example, if you have defined a left unary operator named <literal>@</literal>,
|
|
you cannot write <literal>X*@Y</literal>; you must write
|
|
<literal>X* @Y</literal> to ensure that
|
|
<productname>PostgreSQL</productname> reads it as two operator names
|
|
not one.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2>
|
|
<title>Special Characters</title>
|
|
|
|
<para>
|
|
Some characters that are not alphanumeric have a special meaning
|
|
that is different from being an operator. Details on the usage can
|
|
be found at the location where the respective syntax element is
|
|
described. This section only exists to advise the existence and
|
|
summarize the purposes of these characters.
|
|
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
A dollar sign (<literal>$</literal>) followed by digits is used
|
|
to represent a positional parameter in the body of a function
|
|
definition or a prepared statement. In other contexts the
|
|
dollar sign may be part of an identifier.
|
|
</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>
|
|
Parentheses (<literal>()</literal>) have their usual meaning to
|
|
group expressions and enforce precedence. In some cases
|
|
parentheses are required as part of the fixed syntax of a
|
|
particular SQL command.
|
|
</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>
|
|
Brackets (<literal>[]</literal>) are used to select the elements
|
|
of an array. See <xref linkend="arrays"> for more information
|
|
on arrays.
|
|
</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>
|
|
Commas (<literal>,</literal>) are used in some syntactical
|
|
constructs to separate the elements of a list.
|
|
</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>
|
|
The semicolon (<literal>;</literal>) terminates an SQL command.
|
|
It cannot appear anywhere within a command, except within a
|
|
string constant or quoted identifier.
|
|
</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>
|
|
The colon (<literal>:</literal>) is used to select
|
|
<quote>slices</quote> from arrays. (See <xref
|
|
linkend="arrays">.) In certain SQL dialects (such as Embedded
|
|
SQL), the colon is used to prefix variable names.
|
|
</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>
|
|
The asterisk (<literal>*</literal>) has a special meaning when
|
|
used in the <command>SELECT</command> command or with the
|
|
<function>COUNT</function> aggregate function.
|
|
</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>
|
|
The period (<literal>.</literal>) is used in numeric
|
|
constants, and to separate schema, table, and column names.
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="sql-syntax-comments">
|
|
<title>Comments</title>
|
|
|
|
<indexterm zone="sql-syntax-comments">
|
|
<primary>comments</primary>
|
|
<secondary>in SQL</secondary>
|
|
</indexterm>
|
|
|
|
<para>
|
|
A comment is an arbitrary sequence of characters beginning with
|
|
double dashes and extending to the end of the line, e.g.:
|
|
<programlisting>
|
|
-- This is a standard SQL92 comment
|
|
</programlisting>
|
|
</para>
|
|
|
|
<para>
|
|
Alternatively, C-style block comments can be used:
|
|
<programlisting>
|
|
/* multiline comment
|
|
* with nesting: /* nested block comment */
|
|
*/
|
|
</programlisting>
|
|
where the comment begins with <literal>/*</literal> and extends to
|
|
the matching occurrence of <literal>*/</literal>. These block
|
|
comments nest, as specified in SQL99 but unlike C, so that one can
|
|
comment out larger blocks of code that may contain existing block
|
|
comments.
|
|
</para>
|
|
|
|
<para>
|
|
A comment is removed from the input stream before further syntax
|
|
analysis and is effectively replaced by whitespace.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="sql-precedence">
|
|
<title>Lexical Precedence</title>
|
|
|
|
<indexterm zone="sql-precedence">
|
|
<primary>operators</primary>
|
|
<secondary>precedence</secondary>
|
|
</indexterm>
|
|
|
|
<para>
|
|
<xref linkend="sql-precedence-table"> shows the precedence and
|
|
associativity of the operators in <productname>PostgreSQL</>.
|
|
Most operators have the same precedence and are left-associative.
|
|
The precedence and associativity of the operators is hard-wired
|
|
into the parser. This may lead to non-intuitive behavior; for
|
|
example the Boolean operators <literal><</> and
|
|
<literal>></> have a different precedence than the Boolean
|
|
operators <literal><=</> and <literal>>=</>. Also, you will
|
|
sometimes need to add parentheses when using combinations of
|
|
binary and unary operators. For instance
|
|
<programlisting>
|
|
SELECT 5 ! - 6;
|
|
</programlisting>
|
|
will be parsed as
|
|
<programlisting>
|
|
SELECT 5 ! (- 6);
|
|
</programlisting>
|
|
because the parser has no idea -- until it is too late -- that
|
|
<token>!</token> is defined as a postfix operator, not an infix one.
|
|
To get the desired behavior in this case, you must write
|
|
<programlisting>
|
|
SELECT (5 !) - 6;
|
|
</programlisting>
|
|
This is the price one pays for extensibility.
|
|
</para>
|
|
|
|
<table id="sql-precedence-table">
|
|
<title>Operator Precedence (decreasing)</title>
|
|
|
|
<tgroup cols="3">
|
|
<thead>
|
|
<row>
|
|
<entry>Operator/Element</entry>
|
|
<entry>Associativity</entry>
|
|
<entry>Description</entry>
|
|
</row>
|
|
</thead>
|
|
|
|
<tbody>
|
|
<row>
|
|
<entry><token>.</token></entry>
|
|
<entry>left</entry>
|
|
<entry>table/column name separator</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><token>::</token></entry>
|
|
<entry>left</entry>
|
|
<entry><productname>PostgreSQL</productname>-style typecast</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><token>[</token> <token>]</token></entry>
|
|
<entry>left</entry>
|
|
<entry>array element selection</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><token>-</token></entry>
|
|
<entry>right</entry>
|
|
<entry>unary minus</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><token>^</token></entry>
|
|
<entry>left</entry>
|
|
<entry>exponentiation</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><token>*</token> <token>/</token> <token>%</token></entry>
|
|
<entry>left</entry>
|
|
<entry>multiplication, division, modulo</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><token>+</token> <token>-</token></entry>
|
|
<entry>left</entry>
|
|
<entry>addition, subtraction</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><token>IS</token></entry>
|
|
<entry></entry>
|
|
<entry><literal>IS TRUE</>, <literal>IS FALSE</>, <literal>IS UNKNOWN</>, <literal>IS NULL</></entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><token>ISNULL</token></entry>
|
|
<entry></entry>
|
|
<entry>test for null</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><token>NOTNULL</token></entry>
|
|
<entry></entry>
|
|
<entry>test for not null</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry>(any other)</entry>
|
|
<entry>left</entry>
|
|
<entry>all other native and user-defined operators</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><token>IN</token></entry>
|
|
<entry></entry>
|
|
<entry>set membership</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><token>BETWEEN</token></entry>
|
|
<entry></entry>
|
|
<entry>containment</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><token>OVERLAPS</token></entry>
|
|
<entry></entry>
|
|
<entry>time interval overlap</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><token>LIKE</token> <token>ILIKE</token> <token>SIMILAR</token></entry>
|
|
<entry></entry>
|
|
<entry>string pattern matching</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><token><</token> <token>></token></entry>
|
|
<entry></entry>
|
|
<entry>less than, greater than</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><token>=</token></entry>
|
|
<entry>right</entry>
|
|
<entry>equality, assignment</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><token>NOT</token></entry>
|
|
<entry>right</entry>
|
|
<entry>logical negation</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><token>AND</token></entry>
|
|
<entry>left</entry>
|
|
<entry>logical conjunction</entry>
|
|
</row>
|
|
|
|
<row>
|
|
<entry><token>OR</token></entry>
|
|
<entry>left</entry>
|
|
<entry>logical disjunction</entry>
|
|
</row>
|
|
</tbody>
|
|
</tgroup>
|
|
</table>
|
|
|
|
<para>
|
|
Note that the operator precedence rules also apply to user-defined
|
|
operators that have the same names as the built-in operators
|
|
mentioned above. For example, if you define a
|
|
<quote>+</quote> operator for some custom data type it will have
|
|
the same precedence as the built-in <quote>+</quote> operator, no
|
|
matter what yours does.
|
|
</para>
|
|
|
|
<para>
|
|
When a schema-qualified operator name is used in the
|
|
<literal>OPERATOR</> syntax, as for example in
|
|
<programlisting>
|
|
SELECT 3 OPERATOR(pg_catalog.+) 4;
|
|
</programlisting>
|
|
the <literal>OPERATOR</> construct is taken to have the default precedence
|
|
shown in <xref linkend="sql-precedence-table"> for <quote>any other</> operator. This is true no matter
|
|
which specific operator name appears inside <literal>OPERATOR()</>.
|
|
</para>
|
|
</sect2>
|
|
</sect1>
|
|
|
|
<sect1 id="sql-expressions">
|
|
<title>Value Expressions</title>
|
|
|
|
<para>
|
|
Value expressions are used in a variety of contexts, such
|
|
as in the target list of the <command>SELECT</command> command, as
|
|
new column values in <command>INSERT</command> or
|
|
<command>UPDATE</command>, or in search conditions in a number of
|
|
commands. The result of a value expression is sometimes called a
|
|
<firstterm>scalar</firstterm>, to distinguish it from the result of
|
|
a table expression (which is a table). Value expressions are
|
|
therefore also called <firstterm>scalar expressions</firstterm> (or
|
|
even simply <firstterm>expressions</firstterm>). The expression
|
|
syntax allows the calculation of values from primitive parts using
|
|
arithmetic, logical, set, and other operations.
|
|
</para>
|
|
|
|
<para>
|
|
A value expression is one of the following:
|
|
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
A constant or literal value; see <xref linkend="sql-syntax-constants">.
|
|
</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>
|
|
A column reference.
|
|
</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>
|
|
A positional parameter reference, in the body of a function definition.
|
|
</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>
|
|
An operator invocation.
|
|
</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>
|
|
A function call.
|
|
</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>
|
|
An aggregate expression.
|
|
</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>
|
|
A type cast.
|
|
</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>
|
|
A scalar subquery.
|
|
</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>
|
|
Another value expression in parentheses, useful to group subexpressions and override precedence.
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
|
|
<para>
|
|
In addition to this list, there are a number of constructs that can
|
|
be classified as an expression but do not follow any general syntax
|
|
rules. These generally have the semantics of a function or
|
|
operator and are explained in the appropriate location in <xref
|
|
linkend="functions">. An example is the <literal>IS NULL</literal>
|
|
clause.
|
|
</para>
|
|
|
|
<para>
|
|
We have already discussed constants in <xref
|
|
linkend="sql-syntax-constants">. The following sections discuss
|
|
the remaining options.
|
|
</para>
|
|
|
|
<sect2>
|
|
<title>Column References</title>
|
|
|
|
<para>
|
|
A column can be referenced in the form
|
|
<synopsis>
|
|
<replaceable>correlation</replaceable>.<replaceable>columnname</replaceable>
|
|
</synopsis>
|
|
or
|
|
<synopsis>
|
|
<replaceable>correlation</replaceable>.<replaceable>columnname</replaceable>[<replaceable>subscript</replaceable>]
|
|
</synopsis>
|
|
(Here, the brackets <literal>[ ]</literal> are meant to appear literally.)
|
|
</para>
|
|
|
|
<para>
|
|
<replaceable>correlation</replaceable> is the name of a
|
|
table (possibly qualified), or an alias for a table defined by means of a
|
|
<literal>FROM</literal> clause, or
|
|
the key words <literal>NEW</literal> or <literal>OLD</literal>.
|
|
(<literal>NEW</literal> and <literal>OLD</literal> can only appear in rewrite rules,
|
|
while other correlation names can be used in any SQL statement.)
|
|
The correlation name and separating dot may be omitted if the column name
|
|
is unique across all the tables being used in the current query. (See also <xref linkend="queries">.)
|
|
</para>
|
|
|
|
<para>
|
|
If <replaceable>column</replaceable> is of an array type, then the
|
|
optional <replaceable>subscript</replaceable> selects a specific
|
|
element or elements in the array. If no subscript is provided, then the
|
|
whole array is selected. (See <xref linkend="arrays"> for more about
|
|
arrays.)
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2>
|
|
<title>Positional Parameters</title>
|
|
|
|
<para>
|
|
A positional parameter reference is used to indicate a value
|
|
that is supplied externally to an SQL statement. Parameters are
|
|
used in SQL function definitions and in prepared queries. Some
|
|
client libraries also support specifying data values separately
|
|
from the SQL command string, in which case parameters are used to
|
|
refer to the out-of-line data values.
|
|
The form of a parameter reference is:
|
|
<synopsis>
|
|
$<replaceable>number</replaceable>
|
|
</synopsis>
|
|
</para>
|
|
|
|
<para>
|
|
For example, consider the definition of a function,
|
|
<function>dept</function>, as
|
|
|
|
<programlisting>
|
|
CREATE FUNCTION dept(text) RETURNS dept
|
|
AS 'SELECT * FROM dept WHERE name = $1'
|
|
LANGUAGE SQL;
|
|
</programlisting>
|
|
|
|
Here the <literal>$1</literal> will be replaced by the first
|
|
function argument when the function is invoked.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2>
|
|
<title>Operator Invocations</title>
|
|
|
|
<para>
|
|
There are three possible syntaxes for an operator invocation:
|
|
<simplelist>
|
|
<member><replaceable>expression</replaceable> <replaceable>operator</replaceable> <replaceable>expression</replaceable> (binary infix operator)</member>
|
|
<member><replaceable>operator</replaceable> <replaceable>expression</replaceable> (unary prefix operator)</member>
|
|
<member><replaceable>expression</replaceable> <replaceable>operator</replaceable> (unary postfix operator)</member>
|
|
</simplelist>
|
|
where the <replaceable>operator</replaceable> token follows the syntax
|
|
rules of <xref linkend="sql-syntax-operators">, or is one of the
|
|
keywords <token>AND</token>, <token>OR</token>, and
|
|
<token>NOT</token>, or is a qualified operator name
|
|
<synopsis>
|
|
<literal>OPERATOR(</><replaceable>schema</><literal>.</><replaceable>operatorname</><literal>)</>
|
|
</synopsis>
|
|
Which particular operators exist and whether
|
|
they are unary or binary depends on what operators have been
|
|
defined by the system or the user. <xref linkend="functions">
|
|
describes the built-in operators.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2>
|
|
<title>Function Calls</title>
|
|
|
|
<para>
|
|
The syntax for a function call is the name of a function
|
|
(possibly qualified with a schema name), followed by its argument list
|
|
enclosed in parentheses:
|
|
|
|
<synopsis>
|
|
<replaceable>function</replaceable> (<optional><replaceable>expression</replaceable> <optional>, <replaceable>expression</replaceable> ... </optional></optional> )
|
|
</synopsis>
|
|
</para>
|
|
|
|
<para>
|
|
For example, the following computes the square root of 2:
|
|
<programlisting>
|
|
sqrt(2)
|
|
</programlisting>
|
|
</para>
|
|
|
|
<para>
|
|
The list of built-in functions is in <xref linkend="functions">.
|
|
Other functions may be added by the user.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="syntax-aggregates">
|
|
<title>Aggregate Expressions</title>
|
|
|
|
<indexterm zone="syntax-aggregates">
|
|
<primary>aggregate functions</primary>
|
|
</indexterm>
|
|
|
|
<para>
|
|
An <firstterm>aggregate expression</firstterm> represents the
|
|
application of an aggregate function across the rows selected by a
|
|
query. An aggregate function reduces multiple inputs to a single
|
|
output value, such as the sum or average of the inputs. The
|
|
syntax of an aggregate expression is one of the following:
|
|
|
|
<synopsis>
|
|
<replaceable>aggregate_name</replaceable> (<replaceable>expression</replaceable>)
|
|
<replaceable>aggregate_name</replaceable> (ALL <replaceable>expression</replaceable>)
|
|
<replaceable>aggregate_name</replaceable> (DISTINCT <replaceable>expression</replaceable>)
|
|
<replaceable>aggregate_name</replaceable> ( * )
|
|
</synopsis>
|
|
|
|
where <replaceable>aggregate_name</replaceable> is a previously
|
|
defined aggregate (possibly a qualified name), and
|
|
<replaceable>expression</replaceable> is
|
|
any value expression that does not itself contain an aggregate
|
|
expression.
|
|
</para>
|
|
|
|
<para>
|
|
The first form of aggregate expression invokes the aggregate
|
|
across all input rows for which the given expression yields a
|
|
non-null value. (Actually, it is up to the aggregate function
|
|
whether to ignore null values or not --- but all the standard ones do.)
|
|
The second form is the same as the first, since
|
|
<literal>ALL</literal> is the default. The third form invokes the
|
|
aggregate for all distinct non-null values of the expression found
|
|
in the input rows. The last form invokes the aggregate once for
|
|
each input row regardless of null or non-null values; since no
|
|
particular input value is specified, it is generally only useful
|
|
for the <function>count()</function> aggregate function.
|
|
</para>
|
|
|
|
<para>
|
|
For example, <literal>count(*)</literal> yields the total number
|
|
of input rows; <literal>count(f1)</literal> yields the number of
|
|
input rows in which <literal>f1</literal> is non-null;
|
|
<literal>count(distinct f1)</literal> yields the number of
|
|
distinct non-null values of <literal>f1</literal>.
|
|
</para>
|
|
|
|
<para>
|
|
The predefined aggregate functions are described in <xref
|
|
linkend="functions-aggregate">. Other aggregate functions may be added
|
|
by the user.
|
|
</para>
|
|
|
|
<para>
|
|
An aggregate expression may only appear in the result list or
|
|
<literal>HAVING</> clause of a <command>SELECT</> command.
|
|
It is forbidden in other clauses, such as <literal>WHERE</>,
|
|
because those clauses are logically evaluated before the results
|
|
of aggregates are formed.
|
|
</para>
|
|
|
|
<para>
|
|
When an aggregate expression appears in a subquery (see
|
|
<xref linkend="sql-syntax-scalar-subqueries"> and
|
|
<xref linkend="functions-subquery">), the aggregate is normally
|
|
evaluated over the rows of the subquery. But an exception occurs
|
|
if the aggregate's argument contains only outer-level variables:
|
|
the aggregate then belongs to the nearest such outer level, and is
|
|
evaluated over the rows of that query. The aggregate expression
|
|
as a whole is then an outer reference for the subquery it appears in,
|
|
and acts as a constant over any one evaluation of that subquery.
|
|
The restriction about
|
|
appearing only in the result list or <literal>HAVING</> clause
|
|
applies with respect to the query level that the aggregate belongs to.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="sql-syntax-type-casts">
|
|
<title>Type Casts</title>
|
|
|
|
<indexterm>
|
|
<primary>data types</primary>
|
|
<secondary>type casts</secondary>
|
|
</indexterm>
|
|
|
|
<para>
|
|
A type cast specifies a conversion from one data type to another.
|
|
<productname>PostgreSQL</productname> accepts two equivalent syntaxes
|
|
for type casts:
|
|
<synopsis>
|
|
CAST ( <replaceable>expression</replaceable> AS <replaceable>type</replaceable> )
|
|
<replaceable>expression</replaceable>::<replaceable>type</replaceable>
|
|
</synopsis>
|
|
The <literal>CAST</> syntax conforms to SQL; the syntax with
|
|
<literal>::</literal> is historical <productname>PostgreSQL</productname>
|
|
usage.
|
|
</para>
|
|
|
|
<para>
|
|
When a cast is applied to a value expression of a known type, it
|
|
represents a run-time type conversion. The cast will succeed only
|
|
if a suitable type conversion function is available. Notice that this
|
|
is subtly different from the use of casts with constants, as shown in
|
|
<xref linkend="sql-syntax-constants-generic">. A cast applied to an
|
|
unadorned string literal represents the initial assignment of a type
|
|
to a literal constant value, and so it will succeed for any type
|
|
(if the contents of the string literal are acceptable input syntax for the
|
|
data type).
|
|
</para>
|
|
|
|
<para>
|
|
An explicit type cast may usually be omitted if there is no ambiguity as
|
|
to the type that a value expression must produce (for example, when it is
|
|
assigned to a table column); the system will automatically apply a
|
|
type cast in such cases. However, automatic casting is only done for
|
|
casts that are marked <quote>OK to apply implicitly</>
|
|
in the system catalogs. Other casts must be invoked with
|
|
explicit casting syntax. This restriction is intended to prevent
|
|
surprising conversions from being applied silently.
|
|
</para>
|
|
|
|
<para>
|
|
It is also possible to specify a type cast using a function-like
|
|
syntax:
|
|
<synopsis>
|
|
<replaceable>typename</replaceable> ( <replaceable>expression</replaceable> )
|
|
</synopsis>
|
|
However, this only works for types whose names are also valid as
|
|
function names. For example, <literal>double precision</literal>
|
|
can't be used this way, but the equivalent <literal>float8</literal>
|
|
can. Also, the names <literal>interval</>, <literal>time</>, and
|
|
<literal>timestamp</> can only be used in this fashion if they are
|
|
double-quoted, because of syntactic conflicts. Therefore, the use of
|
|
the function-like cast syntax leads to inconsistencies and should
|
|
probably be avoided in new applications.
|
|
|
|
(The function-like syntax is in fact just a function call. When
|
|
one of the two standard cast syntaxes is used to do a run-time
|
|
conversion, it will internally invoke a registered function to
|
|
perform the conversion. By convention, these conversion functions
|
|
have the same name as their output type, but this is not something
|
|
that a portable application should rely on.)
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="sql-syntax-scalar-subqueries">
|
|
<title>Scalar Subqueries</title>
|
|
|
|
<para>
|
|
A scalar subquery is an ordinary
|
|
<command>SELECT</command> query in parentheses that returns exactly one
|
|
row with one column. (See <xref linkend="queries"> for information about writing queries.)
|
|
The <command>SELECT</command> query is executed
|
|
and the single returned value is used in the surrounding value expression.
|
|
It is an error to use a query that
|
|
returns more than one row or more than one column as a scalar subquery.
|
|
(But if, during a particular execution, the subquery returns no rows,
|
|
there is no error; the scalar result is taken to be null.)
|
|
The subquery can refer to variables from the surrounding query,
|
|
which will act as constants during any one evaluation of the subquery.
|
|
See also <xref linkend="functions-subquery">.
|
|
</para>
|
|
|
|
<para>
|
|
For example, the following finds the largest city population in each
|
|
state:
|
|
<programlisting>
|
|
SELECT name, (SELECT max(pop) FROM cities WHERE cities.state = states.name)
|
|
FROM states;
|
|
</programlisting>
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2>
|
|
<title>Expression Evaluation</title>
|
|
|
|
<para>
|
|
The order of evaluation of subexpressions is not defined. In
|
|
particular, the inputs of an operator or function are not necessarily
|
|
evaluated left-to-right or in any other fixed order.
|
|
</para>
|
|
|
|
<para>
|
|
Furthermore, if the result of an expression can be determined by
|
|
evaluating only some parts of it, then other subexpressions
|
|
might not be evaluated at all. For instance, if one wrote
|
|
<programlisting>
|
|
SELECT true OR somefunc();
|
|
</programlisting>
|
|
then <literal>somefunc()</literal> would (probably) not be called
|
|
at all. The same would be the case if one wrote
|
|
<programlisting>
|
|
SELECT somefunc() OR true;
|
|
</programlisting>
|
|
Note that this is not the same as the left-to-right
|
|
<quote>short-circuiting</quote> of Boolean operators that is found
|
|
in some programming languages.
|
|
</para>
|
|
|
|
<para>
|
|
As a consequence, it is unwise to use functions with side effects
|
|
as part of complex expressions. It is particularly dangerous to
|
|
rely on side effects or evaluation order in <literal>WHERE</> and <literal>HAVING</> clauses,
|
|
since those clauses are extensively reprocessed as part of
|
|
developing an execution plan. Boolean
|
|
expressions (<literal>AND</>/<literal>OR</>/<literal>NOT</> combinations) in those clauses may be reorganized
|
|
in any manner allowed by the laws of Boolean algebra.
|
|
</para>
|
|
|
|
<para>
|
|
When it is essential to force evaluation order, a <literal>CASE</>
|
|
construct (see <xref linkend="functions-conditional">) may be
|
|
used. For example, this is an untrustworthy way of trying to
|
|
avoid division by zero in a <literal>WHERE</> clause:
|
|
<programlisting>
|
|
SELECT ... WHERE x <> 0 AND y/x > 1.5;
|
|
</programlisting>
|
|
But this is safe:
|
|
<programlisting>
|
|
SELECT ... WHERE CASE WHEN x <> 0 THEN y/x > 1.5 ELSE false END;
|
|
</programlisting>
|
|
A <literal>CASE</> construct used in this fashion will defeat optimization attempts,
|
|
so it should only be done when necessary.
|
|
</para>
|
|
</sect2>
|
|
</sect1>
|
|
|
|
</chapter>
|
|
|
|
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|
|
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|
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|
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|
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