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1203 lines
39 KiB
Plaintext
1203 lines
39 KiB
Plaintext
<!--
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$Header: /cvsroot/pgsql/doc/src/sgml/syntax.sgml,v 1.46 2001/09/09 17:21:59 petere 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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<abstract>
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<para>
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A description of the general syntax of SQL.
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</para>
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</abstract>
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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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<informalexample id="sql-syntax-ex-commands">
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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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</informalexample>
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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 the
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<citetitle>Reference Manual</citetitle>.
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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>) or underscore
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(<literal>_</literal>). Subsequent characters in an identifier or
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key word can be letters, digits
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(<literal>0</literal>-<literal>9</literal>), or underscores,
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although the SQL standard will not define a key word that contains
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digits or starts or ends with an underscore.
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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 32 so the maximum identifier length
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is 31 (but at the time the system is built,
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<symbol>NAMEDATALEN</symbol> can be changed 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. This allows constructing table or column names that
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would 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>Postgres</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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<productname>Postgres</productname>' folding of unquoted names to lower
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case 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 four kinds of <firstterm>implicitly typed
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constants</firstterm> in <productname>Postgres</productname>:
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strings, bit strings, integers, and floating point 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>Postgres</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.
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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>. Bit string constants 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>Integer Constants</title>
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<para>
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Integer constants in SQL are sequences of decimal digits (0
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though 9) with no decimal point. The range of legal values
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depends on which integer data type is used, but the plain
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<type>integer</type> type accepts values ranging from -2147483648
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to +2147483647. (The optional plus or minus sign is actually a
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separate unary operator and not part of the integer constant.)
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</para>
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</sect3>
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<sect3>
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<title>Floating Point Constants</title>
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<indexterm>
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<primary>floating point</primary>
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<secondary>constants</secondary>
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</indexterm>
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<para>
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Floating point constants are accepted in these general forms:
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<synopsis>
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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. At least one digit must be before or after the decimal
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point, and after the <literal>e</literal> if you use that option.
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Thus, a floating point constant is distinguished from an integer
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constant by the presence of either the decimal point or the
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exponent clause (or both). There must not be a space or other
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characters embedded in the constant.
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</para>
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<informalexample>
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<para>
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These are some examples of valid floating point constants:
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<literallayout>
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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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</informalexample>
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<para>
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Floating point constants are of type <type>DOUBLE
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PRECISION</type>. <type>REAL</type> can be specified explicitly
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by using <acronym>SQL</acronym> string notation or
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<productname>Postgres</productname> type notation:
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<programlisting>
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REAL '1.23' -- string style
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'1.23'::REAL -- Postgres (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 value inside the string 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
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syntax:
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<synopsis>
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<replaceable>typename</replaceable> ( <replaceable>value</replaceable> )
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</synopsis>
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although this only works for types whose names are also valid as
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function names. (For example, <literal>double precision</literal>
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can't be used this way --- but the equivalent <literal>float8</literal>
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can.)
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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 the type of
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arbitrary expressions, but the form
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<replaceable>type</replaceable>
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'<replaceable>string</replaceable>' can only be used to specify
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the type of a literal constant.
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</para>
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</sect3>
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<sect3>
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<title>Array constants</title>
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<indexterm>
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<primary>arrays</primary>
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<secondary>constants</secondary>
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</indexterm>
|
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<para>
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The general format of an array constant is the following:
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<synopsis>
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'{ <replaceable>val1</replaceable> <replaceable>delim</replaceable> <replaceable>val2</replaceable> <replaceable>delim</replaceable> ... }'
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</synopsis>
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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
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",".) Each <replaceable>val</replaceable> is either a constant
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|
of the array element type, or a sub-array. An example of an
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|
array constant is
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<programlisting>
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'{{1,2,3},{4,5,6},{7,8,9}}'
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</programlisting>
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This constant is a two-dimensional, 3 by 3 array consisting of three
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sub-arrays of integers.
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|
</para>
|
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|
|
<para>
|
|
Individual array elements can be placed between double-quote
|
|
marks (<literal>"</literal>) <!-- " --> to avoid ambiguity
|
|
problems with respect to white space. Without quote marks, the
|
|
array-value parser will skip leading white space.
|
|
</para>
|
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|
|
<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>
|
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|
|
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|
<sect2 id="sql-syntax-operators">
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|
<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
|
|
(31 by default) characters from the following list:
|
|
<literallayout>
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|
+ - * / < > = ~ ! @ # % ^ & | ` ? $
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|
</literallayout>
|
|
|
|
There are a few restrictions on operator names, however:
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|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
"$" (dollar) cannot be a single-character operator, although it
|
|
can be part of a multiple-character operator name.
|
|
</para>
|
|
</listitem>
|
|
|
|
<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 "+" or "-",
|
|
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>Postgres</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 "@",
|
|
you cannot write <literal>X*@Y</literal>; you must write
|
|
<literal>X* @Y</literal> to ensure that
|
|
<productname>Postgres</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 the positional parameters in the body of a function
|
|
definition. In other contexts the dollar sign may be part of an
|
|
operator name.
|
|
</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 floating point
|
|
constants, and to separate 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>
|
|
</sect1>
|
|
|
|
|
|
<sect1 id="sql-syntax-columns">
|
|
<title>Columns</title>
|
|
|
|
<para>
|
|
A <firstterm>column</firstterm>
|
|
is either a user-defined column of a given table or one of the
|
|
following system-defined columns:
|
|
|
|
<indexterm>
|
|
<primary>columns</primary>
|
|
<secondary>system columns</secondary>
|
|
</indexterm>
|
|
|
|
<variablelist>
|
|
<varlistentry>
|
|
<term><structfield>oid</></term>
|
|
<listitem>
|
|
<para>
|
|
<indexterm>
|
|
<primary>OID</primary>
|
|
</indexterm>
|
|
The object identifier (object ID) of a row. This is a serial number
|
|
that is automatically added by Postgres to all table rows (unless
|
|
the table was created WITHOUT OIDS, in which case this column is
|
|
not present).
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><structfield>tableoid</></term>
|
|
<listitem>
|
|
<para>
|
|
The OID of the table containing this row. This attribute is
|
|
particularly handy for queries that select from inheritance
|
|
hierarchies, since without it, it's difficult to tell which
|
|
individual table a row came from. The
|
|
<structfield>tableoid</structfield> can be joined against the
|
|
<structfield>oid</structfield> column of
|
|
<classname>pg_class</classname> to obtain the table name.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><structfield>xmin</></term>
|
|
<listitem>
|
|
<para>
|
|
The identity (transaction ID) of the inserting transaction for
|
|
this tuple. (Note: a tuple is an individual state of a row;
|
|
each UPDATE of a row creates a new tuple for the same logical row.)
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><structfield>cmin</></term>
|
|
<listitem>
|
|
<para>
|
|
The command identifier (starting at zero) within the inserting
|
|
transaction.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><structfield>xmax</></term>
|
|
<listitem>
|
|
<para>
|
|
The identity (transaction ID) of the deleting transaction,
|
|
or zero for an undeleted tuple. It is possible for this field
|
|
to be nonzero in a visible tuple: that usually indicates that the
|
|
deleting transaction hasn't committed yet, or that an attempted
|
|
deletion was rolled back.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><structfield>cmax</></term>
|
|
<listitem>
|
|
<para>
|
|
The command identifier within the deleting transaction, or zero.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><structfield>ctid</></term>
|
|
<listitem>
|
|
<para>
|
|
The tuple ID of the tuple within its table. This is a pair
|
|
(block number, tuple index within block) that identifies the
|
|
physical location of the tuple. Note that although the <structfield>ctid</structfield>
|
|
can be used to locate the tuple very quickly, a row's <structfield>ctid</structfield>
|
|
will change each time it is updated or moved by <command>VACUUM
|
|
FULL</>.
|
|
Therefore <structfield>ctid</structfield> is useless as a long-term row identifier.
|
|
The OID, or even better a user-defined serial number, should
|
|
be used to identify logical rows.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
</variablelist>
|
|
</para>
|
|
|
|
<para>
|
|
OIDs are 32-bit quantities and are assigned from a single cluster-wide
|
|
counter. In a large or long-lived database, it is possible for the
|
|
counter to wrap around. Hence, it is bad practice to assume that OIDs
|
|
are unique, unless you take steps to ensure that they are unique.
|
|
Recommended practice when using OIDs for row identification is to create
|
|
a unique index on the OID column of each table for which the OID will be
|
|
used. Never assume that OIDs are unique across tables; use the
|
|
combination of <structfield>tableoid</> and row OID if you need a database-wide
|
|
identifier. (Future releases of Postgres are likely to use a separate
|
|
OID counter for each table, so that <structfield>tableoid</> <emphasis>must</> be
|
|
included to arrive at a globally unique identifier.)
|
|
</para>
|
|
|
|
<para>
|
|
Transaction identifiers are 32-bit quantities. In a long-lived
|
|
database it is possible for transaction IDs to wrap around. This
|
|
is not a fatal problem given appropriate maintenance procedures;
|
|
see the Administrator's Guide for details. However, it is unwise
|
|
to depend on uniqueness of transaction IDs over the long term
|
|
(more than one billion transactions).
|
|
</para>
|
|
|
|
<para>
|
|
Command identifiers are also 32-bit quantities. This creates a hard
|
|
limit of 2^32 (4 billion) SQL commands within a single transaction.
|
|
In practice this limit is not a problem --- note that the limit is on
|
|
number of SQL queries, not number of tuples processed.
|
|
</para>
|
|
|
|
<para>
|
|
For further information on the system attributes consult
|
|
<xref linkend="STON87a" endterm="STON87a">.
|
|
</para>
|
|
|
|
</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>
|
|
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 <replaceable>operator</replaceable> follows the syntax
|
|
rules of <xref linkend="sql-syntax-operators"> or is one of the
|
|
tokens <token>AND</token>, <token>OR</token>, and
|
|
<token>NOT</token>. 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>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>
|
|
<synopsis>( <replaceable>expression</replaceable> )</synopsis>
|
|
Parentheses are used to group subexpressions and override precedence.
|
|
</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>
|
|
A positional parameter reference, in the body of a function declaration.
|
|
</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>
|
|
A function call
|
|
</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>
|
|
An aggregate expression
|
|
</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>
|
|
A scalar subquery. This is an ordinary
|
|
<command>SELECT</command> in parentheses that returns exactly one
|
|
row with one column. It is an error to use a subquery that
|
|
returns more than one row or more than one column in the context
|
|
of a value expression.
|
|
</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> `['<replaceable>subscript</replaceable>`]'
|
|
</synopsis>
|
|
|
|
<replaceable>correlation</replaceable> is either the name of a
|
|
table, an alias for a table defined by means of a FROM clause, or
|
|
the keyword <literal>NEW</literal> or <literal>OLD</literal>.
|
|
(NEW and OLD can only appear in the action portion of a rule,
|
|
while other correlation names can be used in any SQL statement.)
|
|
The correlation name can be omitted if the column name is unique
|
|
across all the tables being used in the current query. If
|
|
<replaceable>column</replaceable> is of an array type, then the
|
|
optional <replaceable>subscript</replaceable> selects a specific
|
|
element in the array. If no subscript is provided, then the whole
|
|
array is selected. Refer to the description of the particular
|
|
commands in the <citetitle>PostgreSQL Reference Manual</citetitle>
|
|
for the allowed syntax in each case.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2>
|
|
<title>Positional Parameters</title>
|
|
|
|
<para>
|
|
A positional parameter reference is used to indicate a parameter
|
|
in an SQL function. Typically this is used in SQL function
|
|
definition statements. The form of a parameter 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>Function Calls</title>
|
|
|
|
<para>
|
|
The syntax for a function call is the name of a function
|
|
(which is subject to the syntax rules for identifiers of <xref
|
|
linkend="sql-syntax-identifiers">), 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:
|
|
|
|
<simplelist>
|
|
<member><replaceable>aggregate_name</replaceable> (<replaceable>expression</replaceable>)</member>
|
|
<member><replaceable>aggregate_name</replaceable> (ALL <replaceable>expression</replaceable>)</member>
|
|
<member><replaceable>aggregate_name</replaceable> (DISTINCT <replaceable>expression</replaceable>)</member>
|
|
<member><replaceable>aggregate_name</replaceable> ( * )</member>
|
|
</simplelist>
|
|
|
|
where <replaceable>aggregate_name</replaceable> is a previously
|
|
defined aggregate, and <replaceable>expression</replaceable> is
|
|
any 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 NULLs 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>
|
|
</sect2>
|
|
|
|
</sect1>
|
|
|
|
|
|
<sect1 id="sql-precedence">
|
|
<title>Lexical Precedence</title>
|
|
|
|
<indexterm zone="sql-precedence">
|
|
<primary>operators</primary>
|
|
<secondary>precedence</secondary>
|
|
</indexterm>
|
|
|
|
<para>
|
|
The precedence and associativity of the operators is hard-wired
|
|
into the parser. Most operators have the same precedence and are
|
|
left-associative. This may lead to non-intuitive behavior; for
|
|
example the Boolean operators "<" and ">" have a different
|
|
precedence than the Boolean operators "<=" and ">=". 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 tocentry="1">
|
|
<title>Operator Precedence (decreasing)</title>
|
|
|
|
<tgroup cols="2">
|
|
<thead>
|
|
<row>
|
|
<entry>Operator/Element</entry>
|
|
<entry>Associativity</entry>
|
|
<entry>Description</entry>
|
|
</row>
|
|
</thead>
|
|
|
|
<tbody>
|
|
<row>
|
|
<entry><token>::</token></entry>
|
|
<entry>left</entry>
|
|
<entry><productname>Postgres</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>left</entry>
|
|
<entry>table/column name separator</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>test for TRUE, FALSE, UNKNOWN, 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></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>
|
|
</sect1>
|
|
|
|
</chapter>
|
|
|
|
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|
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