91 lines
3.9 KiB
Plaintext
91 lines
3.9 KiB
Plaintext
<!-- doc/src/sgml/keywords.sgml -->
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<appendix id="sql-keywords-appendix">
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<title><acronym>SQL</acronym> Key Words</title>
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<indexterm zone="sql-keywords-appendix">
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<primary>key word</primary>
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<secondary>list of</secondary>
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</indexterm>
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<para>
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<xref linkend="keywords-table"/> lists all tokens that are key words
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in the SQL standard and in <productname>PostgreSQL</productname>
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&version;. Background information can be found in <xref
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linkend="sql-syntax-identifiers"/>.
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(For space reasons, only the latest two versions of the SQL standard, and
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SQL-92 for historical comparison, are included. The differences between
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those and the other intermediate standard versions are small.)
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</para>
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<para>
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SQL distinguishes between <firstterm>reserved</firstterm> and
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<firstterm>non-reserved</firstterm> key words. According to the standard,
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reserved key words
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are the only real key words; they are never allowed as identifiers.
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Non-reserved key words only have a special meaning in particular
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contexts and can be used as identifiers in other contexts. Most
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non-reserved key words are actually the names of built-in tables
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and functions specified by SQL. The concept of non-reserved key
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words essentially only exists to declare that some predefined meaning
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is attached to a word in some contexts.
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</para>
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<para>
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In the <productname>PostgreSQL</productname> parser, life is a bit
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more complicated. There are several different classes of tokens
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ranging from those that can never be used as an identifier to those
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that have absolutely no special status in the parser, but are considered
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ordinary identifiers. (The latter is usually the case for
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functions specified by SQL.) Even reserved key words are not
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completely reserved in <productname>PostgreSQL</productname>, but
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can be used as column labels (for example, <literal>SELECT 55 AS
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CHECK</literal>, even though <token>CHECK</token> is a reserved key
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word).
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</para>
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<para>
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In <xref linkend="keywords-table"/> in the column for
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<productname>PostgreSQL</productname> we classify as
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<quote>non-reserved</quote> those key words that are explicitly
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known to the parser but are allowed as column or table names.
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Some key words that are otherwise
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non-reserved cannot be used as function or data type names and are
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marked accordingly. (Most of these words represent built-in
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functions or data types with special syntax. The function or type
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is still available but it cannot be redefined by the user.) Labeled
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<quote>reserved</quote> are those tokens that are not allowed as
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column or table names. Some reserved key words are
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allowable as names for functions or data types; this is also shown in the
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table. If not so marked, a reserved key word is only allowed as a
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column label.
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A blank entry in this column means that the word is treated as an
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ordinary identifier by <productname>PostgreSQL</productname>.
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</para>
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<para>
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Furthermore, while most key words can be used as <quote>bare</quote>
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column labels without writing <literal>AS</literal> before them (as
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described in <xref linkend="queries-column-labels"/>), there are a few
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that require a leading <literal>AS</literal> to avoid ambiguity. These
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are marked in the table as <quote>requires <literal>AS</literal></quote>.
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</para>
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<para>
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As a general rule, if you get spurious parser errors for commands
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that use any of the listed key words as an identifier, you should
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try quoting the identifier to see if the problem goes away.
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</para>
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<para>
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It is important to understand before studying <xref
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linkend="keywords-table"/> that the fact that a key word is not
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reserved in <productname>PostgreSQL</productname> does not mean that
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the feature related to the word is not implemented. Conversely, the
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presence of a key word does not indicate the existence of a feature.
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</para>
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&keywords-table;
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</appendix>
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