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Diffstat (limited to 'doc/src/sgml/queries.sgml')
| -rw-r--r-- | doc/src/sgml/queries.sgml | 26 |
1 files changed, 15 insertions, 11 deletions
diff --git a/doc/src/sgml/queries.sgml b/doc/src/sgml/queries.sgml index 67ca71e5649..0a643ef5970 100644 --- a/doc/src/sgml/queries.sgml +++ b/doc/src/sgml/queries.sgml @@ -1496,21 +1496,25 @@ SELECT a AS value, b + c AS sum FROM ... </para> <para> - The <literal>AS</literal> keyword is optional, but only if the new column - name does not match any - <productname>PostgreSQL</productname> keyword (see <xref - linkend="sql-keywords-appendix"/>). To avoid an accidental match to - a keyword, you can double-quote the column name. For example, - <literal>VALUE</literal> is a keyword, so this does not work: + The <literal>AS</literal> key word is usually optional, but in some + cases where the desired column name matches a + <productname>PostgreSQL</productname> key word, you must write + <literal>AS</literal> or double-quote the column name in order to + avoid ambiguity. + (<xref linkend="sql-keywords-appendix"/> shows which key words + require <literal>AS</literal> to be used as a column label.) + For example, <literal>FROM</literal> is one such key word, so this + does not work: <programlisting> -SELECT a value, b + c AS sum FROM ... +SELECT a from, b + c AS sum FROM ... </programlisting> - but this does: + but either of these do: <programlisting> -SELECT a "value", b + c AS sum FROM ... +SELECT a AS from, b + c AS sum FROM ... +SELECT a "from", b + c AS sum FROM ... </programlisting> - For protection against possible - future keyword additions, it is recommended that you always either + For greatest safety against possible + future key word additions, it is recommended that you always either write <literal>AS</literal> or double-quote the output column name. </para> |