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func.sgml has grown over the years to the point where it is very difficult to manage. This commit splits out each sect1 piece into its own file, which is then included in the main file, so that the built documentation should be identical to the pre-split documentation. All these new files are placed in a new "func" subdirectory, and the previous func.sgml is removed. Done using scripts developed by: Author: jian he <jian.universality@gmail.com> Discussion: https://postgr.es/m/CACJufxFgAh1--EMwOjMuANe=VTmjkNaZjH+AzSe04-8ZCGiESA@mail.gmail.com
350 lines
13 KiB
Plaintext
350 lines
13 KiB
Plaintext
<sect1 id="functions-subquery">
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<title>Subquery Expressions</title>
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<indexterm>
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<primary>EXISTS</primary>
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</indexterm>
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<indexterm>
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<primary>IN</primary>
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</indexterm>
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<indexterm>
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<primary>NOT IN</primary>
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</indexterm>
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<indexterm>
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<primary>ANY</primary>
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</indexterm>
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<indexterm>
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<primary>ALL</primary>
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</indexterm>
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<indexterm>
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<primary>SOME</primary>
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</indexterm>
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<indexterm>
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<primary>subquery</primary>
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</indexterm>
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<para>
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This section describes the <acronym>SQL</acronym>-compliant subquery
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expressions available in <productname>PostgreSQL</productname>.
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All of the expression forms documented in this section return
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Boolean (true/false) results.
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</para>
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<sect2 id="functions-subquery-exists">
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<title><literal>EXISTS</literal></title>
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<synopsis>
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EXISTS (<replaceable>subquery</replaceable>)
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</synopsis>
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<para>
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The argument of <token>EXISTS</token> is an arbitrary <command>SELECT</command> statement,
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or <firstterm>subquery</firstterm>. The
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subquery is evaluated to determine whether it returns any rows.
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If it returns at least one row, the result of <token>EXISTS</token> is
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<quote>true</quote>; if the subquery returns no rows, the result of <token>EXISTS</token>
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is <quote>false</quote>.
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</para>
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<para>
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The subquery can refer to variables from the surrounding query,
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which will act as constants during any one evaluation of the subquery.
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</para>
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<para>
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The subquery will generally only be executed long enough to determine
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whether at least one row is returned, not all the way to completion.
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It is unwise to write a subquery that has side effects (such as
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calling sequence functions); whether the side effects occur
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might be unpredictable.
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</para>
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<para>
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Since the result depends only on whether any rows are returned,
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and not on the contents of those rows, the output list of the
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subquery is normally unimportant. A common coding convention is
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to write all <literal>EXISTS</literal> tests in the form
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<literal>EXISTS(SELECT 1 WHERE ...)</literal>. There are exceptions to
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this rule however, such as subqueries that use <token>INTERSECT</token>.
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</para>
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<para>
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This simple example is like an inner join on <literal>col2</literal>, but
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it produces at most one output row for each <literal>tab1</literal> row,
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even if there are several matching <literal>tab2</literal> rows:
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<screen>
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SELECT col1
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FROM tab1
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WHERE EXISTS (SELECT 1 FROM tab2 WHERE col2 = tab1.col2);
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</screen>
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</para>
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</sect2>
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<sect2 id="functions-subquery-in">
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<title><literal>IN</literal></title>
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<synopsis>
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<replaceable>expression</replaceable> IN (<replaceable>subquery</replaceable>)
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</synopsis>
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<para>
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The right-hand side is a parenthesized
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subquery, which must return exactly one column. The left-hand expression
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is evaluated and compared to each row of the subquery result.
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The result of <token>IN</token> is <quote>true</quote> if any equal subquery row is found.
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The result is <quote>false</quote> if no equal row is found (including the
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case where the subquery returns no rows).
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</para>
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<para>
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Note that if the left-hand expression yields null, or if there are
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no equal right-hand values and at least one right-hand row yields
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null, the result of the <token>IN</token> construct will be null, not false.
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This is in accordance with SQL's normal rules for Boolean combinations
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of null values.
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</para>
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<para>
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As with <token>EXISTS</token>, it's unwise to assume that the subquery will
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be evaluated completely.
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</para>
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<synopsis>
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<replaceable>row_constructor</replaceable> IN (<replaceable>subquery</replaceable>)
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</synopsis>
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<para>
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The left-hand side of this form of <token>IN</token> is a row constructor,
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as described in <xref linkend="sql-syntax-row-constructors"/>.
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The right-hand side is a parenthesized
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subquery, which must return exactly as many columns as there are
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expressions in the left-hand row. The left-hand expressions are
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evaluated and compared row-wise to each row of the subquery result.
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The result of <token>IN</token> is <quote>true</quote> if any equal subquery row is found.
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The result is <quote>false</quote> if no equal row is found (including the
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case where the subquery returns no rows).
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</para>
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<para>
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As usual, null values in the rows are combined per
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the normal rules of SQL Boolean expressions. Two rows are considered
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equal if all their corresponding members are non-null and equal; the rows
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are unequal if any corresponding members are non-null and unequal;
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otherwise the result of that row comparison is unknown (null).
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If all the per-row results are either unequal or null, with at least one
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null, then the result of <token>IN</token> is null.
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</para>
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</sect2>
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<sect2 id="functions-subquery-notin">
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<title><literal>NOT IN</literal></title>
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<synopsis>
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<replaceable>expression</replaceable> NOT IN (<replaceable>subquery</replaceable>)
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</synopsis>
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<para>
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The right-hand side is a parenthesized
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subquery, which must return exactly one column. The left-hand expression
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is evaluated and compared to each row of the subquery result.
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The result of <token>NOT IN</token> is <quote>true</quote> if only unequal subquery rows
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are found (including the case where the subquery returns no rows).
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The result is <quote>false</quote> if any equal row is found.
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</para>
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<para>
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Note that if the left-hand expression yields null, or if there are
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no equal right-hand values and at least one right-hand row yields
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null, the result of the <token>NOT IN</token> construct will be null, not true.
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This is in accordance with SQL's normal rules for Boolean combinations
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of null values.
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</para>
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<para>
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As with <token>EXISTS</token>, it's unwise to assume that the subquery will
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be evaluated completely.
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</para>
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<synopsis>
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<replaceable>row_constructor</replaceable> NOT IN (<replaceable>subquery</replaceable>)
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</synopsis>
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<para>
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The left-hand side of this form of <token>NOT IN</token> is a row constructor,
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as described in <xref linkend="sql-syntax-row-constructors"/>.
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The right-hand side is a parenthesized
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subquery, which must return exactly as many columns as there are
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expressions in the left-hand row. The left-hand expressions are
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evaluated and compared row-wise to each row of the subquery result.
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The result of <token>NOT IN</token> is <quote>true</quote> if only unequal subquery rows
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are found (including the case where the subquery returns no rows).
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The result is <quote>false</quote> if any equal row is found.
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</para>
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<para>
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As usual, null values in the rows are combined per
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the normal rules of SQL Boolean expressions. Two rows are considered
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equal if all their corresponding members are non-null and equal; the rows
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are unequal if any corresponding members are non-null and unequal;
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otherwise the result of that row comparison is unknown (null).
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If all the per-row results are either unequal or null, with at least one
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null, then the result of <token>NOT IN</token> is null.
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</para>
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</sect2>
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<sect2 id="functions-subquery-any-some">
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<title><literal>ANY</literal>/<literal>SOME</literal></title>
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<synopsis>
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<replaceable>expression</replaceable> <replaceable>operator</replaceable> ANY (<replaceable>subquery</replaceable>)
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<replaceable>expression</replaceable> <replaceable>operator</replaceable> SOME (<replaceable>subquery</replaceable>)
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</synopsis>
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<para>
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The right-hand side is a parenthesized
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subquery, which must return exactly one column. The left-hand expression
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is evaluated and compared to each row of the subquery result using the
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given <replaceable>operator</replaceable>, which must yield a Boolean
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result.
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The result of <token>ANY</token> is <quote>true</quote> if any true result is obtained.
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The result is <quote>false</quote> if no true result is found (including the
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case where the subquery returns no rows).
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</para>
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<para>
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<token>SOME</token> is a synonym for <token>ANY</token>.
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<token>IN</token> is equivalent to <literal>= ANY</literal>.
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</para>
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<para>
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Note that if there are no successes and at least one right-hand row yields
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null for the operator's result, the result of the <token>ANY</token> construct
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will be null, not false.
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This is in accordance with SQL's normal rules for Boolean combinations
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of null values.
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</para>
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<para>
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As with <token>EXISTS</token>, it's unwise to assume that the subquery will
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be evaluated completely.
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</para>
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<synopsis>
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<replaceable>row_constructor</replaceable> <replaceable>operator</replaceable> ANY (<replaceable>subquery</replaceable>)
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<replaceable>row_constructor</replaceable> <replaceable>operator</replaceable> SOME (<replaceable>subquery</replaceable>)
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</synopsis>
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<para>
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The left-hand side of this form of <token>ANY</token> is a row constructor,
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as described in <xref linkend="sql-syntax-row-constructors"/>.
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The right-hand side is a parenthesized
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subquery, which must return exactly as many columns as there are
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expressions in the left-hand row. The left-hand expressions are
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evaluated and compared row-wise to each row of the subquery result,
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using the given <replaceable>operator</replaceable>.
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The result of <token>ANY</token> is <quote>true</quote> if the comparison
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returns true for any subquery row.
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The result is <quote>false</quote> if the comparison returns false for every
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subquery row (including the case where the subquery returns no
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rows).
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The result is NULL if no comparison with a subquery row returns true,
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and at least one comparison returns NULL.
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</para>
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<para>
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See <xref linkend="row-wise-comparison"/> for details about the meaning
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of a row constructor comparison.
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</para>
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</sect2>
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<sect2 id="functions-subquery-all">
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<title><literal>ALL</literal></title>
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<synopsis>
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<replaceable>expression</replaceable> <replaceable>operator</replaceable> ALL (<replaceable>subquery</replaceable>)
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</synopsis>
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<para>
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The right-hand side is a parenthesized
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subquery, which must return exactly one column. The left-hand expression
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is evaluated and compared to each row of the subquery result using the
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given <replaceable>operator</replaceable>, which must yield a Boolean
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result.
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The result of <token>ALL</token> is <quote>true</quote> if all rows yield true
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(including the case where the subquery returns no rows).
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The result is <quote>false</quote> if any false result is found.
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The result is NULL if no comparison with a subquery row returns false,
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and at least one comparison returns NULL.
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</para>
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<para>
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<token>NOT IN</token> is equivalent to <literal><> ALL</literal>.
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</para>
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<para>
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As with <token>EXISTS</token>, it's unwise to assume that the subquery will
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be evaluated completely.
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</para>
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<synopsis>
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<replaceable>row_constructor</replaceable> <replaceable>operator</replaceable> ALL (<replaceable>subquery</replaceable>)
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</synopsis>
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<para>
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The left-hand side of this form of <token>ALL</token> is a row constructor,
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as described in <xref linkend="sql-syntax-row-constructors"/>.
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The right-hand side is a parenthesized
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subquery, which must return exactly as many columns as there are
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expressions in the left-hand row. The left-hand expressions are
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evaluated and compared row-wise to each row of the subquery result,
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using the given <replaceable>operator</replaceable>.
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The result of <token>ALL</token> is <quote>true</quote> if the comparison
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returns true for all subquery rows (including the
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case where the subquery returns no rows).
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The result is <quote>false</quote> if the comparison returns false for any
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subquery row.
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The result is NULL if no comparison with a subquery row returns false,
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and at least one comparison returns NULL.
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</para>
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<para>
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See <xref linkend="row-wise-comparison"/> for details about the meaning
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of a row constructor comparison.
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</para>
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</sect2>
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<sect2 id="functions-subquery-single-row-comp">
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<title>Single-Row Comparison</title>
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<indexterm zone="functions-subquery">
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<primary>comparison</primary>
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<secondary>subquery result row</secondary>
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</indexterm>
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<synopsis>
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<replaceable>row_constructor</replaceable> <replaceable>operator</replaceable> (<replaceable>subquery</replaceable>)
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</synopsis>
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<para>
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The left-hand side is a row constructor,
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as described in <xref linkend="sql-syntax-row-constructors"/>.
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The right-hand side is a parenthesized subquery, which must return exactly
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as many columns as there are expressions in the left-hand row. Furthermore,
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the subquery cannot return more than one row. (If it returns zero rows,
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the result is taken to be null.) The left-hand side is evaluated and
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compared row-wise to the single subquery result row.
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</para>
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<para>
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See <xref linkend="row-wise-comparison"/> for details about the meaning
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of a row constructor comparison.
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</para>
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</sect2>
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</sect1>
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