635 lines
24 KiB
Plaintext
635 lines
24 KiB
Plaintext
<!--
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doc/src/sgml/ref/merge.sgml
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PostgreSQL documentation
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-->
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<refentry id="sql-merge">
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<indexterm zone="sql-merge">
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<primary>MERGE</primary>
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</indexterm>
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<refmeta>
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<refentrytitle>MERGE</refentrytitle>
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<manvolnum>7</manvolnum>
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<refmiscinfo>SQL - Language Statements</refmiscinfo>
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</refmeta>
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<refnamediv>
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<refname>MERGE</refname>
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<refpurpose>conditionally insert, update, or delete rows of a table</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<synopsis>
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[ WITH <replaceable class="parameter">with_query</replaceable> [, ...] ]
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MERGE INTO [ ONLY ] <replaceable class="parameter">target_table_name</replaceable> [ * ] [ [ AS ] <replaceable class="parameter">target_alias</replaceable> ]
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USING <replaceable class="parameter">data_source</replaceable> ON <replaceable class="parameter">join_condition</replaceable>
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<replaceable class="parameter">when_clause</replaceable> [...]
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<phrase>where <replaceable class="parameter">data_source</replaceable> is:</phrase>
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{ [ ONLY ] <replaceable class="parameter">source_table_name</replaceable> [ * ] | ( <replaceable class="parameter">source_query</replaceable> ) } [ [ AS ] <replaceable class="parameter">source_alias</replaceable> ]
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<phrase>and <replaceable class="parameter">when_clause</replaceable> is:</phrase>
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{ WHEN MATCHED [ AND <replaceable class="parameter">condition</replaceable> ] THEN { <replaceable class="parameter">merge_update</replaceable> | <replaceable class="parameter">merge_delete</replaceable> | DO NOTHING } |
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WHEN NOT MATCHED [ AND <replaceable class="parameter">condition</replaceable> ] THEN { <replaceable class="parameter">merge_insert</replaceable> | DO NOTHING } }
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<phrase>and <replaceable class="parameter">merge_insert</replaceable> is:</phrase>
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INSERT [( <replaceable class="parameter">column_name</replaceable> [, ...] )]
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[ OVERRIDING { SYSTEM | USER } VALUE ]
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{ VALUES ( { <replaceable class="parameter">expression</replaceable> | DEFAULT } [, ...] ) | DEFAULT VALUES }
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<phrase>and <replaceable class="parameter">merge_update</replaceable> is:</phrase>
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UPDATE SET { <replaceable class="parameter">column_name</replaceable> = { <replaceable class="parameter">expression</replaceable> | DEFAULT } |
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( <replaceable class="parameter">column_name</replaceable> [, ...] ) = ( { <replaceable class="parameter">expression</replaceable> | DEFAULT } [, ...] ) } [, ...]
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<phrase>and <replaceable class="parameter">merge_delete</replaceable> is:</phrase>
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DELETE
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</synopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para>
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<command>MERGE</command> performs actions that modify rows in the
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<replaceable class="parameter">target_table_name</replaceable>,
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using the <replaceable class="parameter">data_source</replaceable>.
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<command>MERGE</command> provides a single <acronym>SQL</acronym>
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statement that can conditionally <command>INSERT</command>,
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<command>UPDATE</command> or <command>DELETE</command> rows, a task
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that would otherwise require multiple procedural language statements.
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</para>
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<para>
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First, the <command>MERGE</command> command performs a join
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from <replaceable class="parameter">data_source</replaceable> to
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<replaceable class="parameter">target_table_name</replaceable>
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producing zero or more candidate change rows. For each candidate change
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row, the status of <literal>MATCHED</literal> or <literal>NOT MATCHED</literal>
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is set just once, after which <literal>WHEN</literal> clauses are evaluated
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in the order specified. For each candidate change row, the first clause to
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evaluate as true is executed. No more than one <literal>WHEN</literal>
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clause is executed for any candidate change row.
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</para>
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<para>
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<command>MERGE</command> actions have the same effect as
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regular <command>UPDATE</command>, <command>INSERT</command>, or
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<command>DELETE</command> commands of the same names. The syntax of
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those commands is different, notably that there is no <literal>WHERE</literal>
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clause and no table name is specified. All actions refer to the
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<replaceable class="parameter">target_table_name</replaceable>,
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though modifications to other tables may be made using triggers.
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</para>
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<para>
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When <literal>DO NOTHING</literal> is specified, the source row is
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skipped. Since actions are evaluated in their specified order, <literal>DO
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NOTHING</literal> can be handy to skip non-interesting source rows before
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more fine-grained handling.
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</para>
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<para>
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There is no separate <literal>MERGE</literal> privilege.
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If you specify an update action, you must have the
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<literal>UPDATE</literal> privilege on the column(s)
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of the <replaceable class="parameter">target_table_name</replaceable>
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that are referred to in the <literal>SET</literal> clause.
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If you specify an insert action, you must have the <literal>INSERT</literal>
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privilege on the <replaceable class="parameter">target_table_name</replaceable>.
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If you specify an delete action, you must have the <literal>DELETE</literal>
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privilege on the <replaceable class="parameter">target_table_name</replaceable>.
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Privileges are tested once at statement start and are checked
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whether or not particular <literal>WHEN</literal> clauses are executed.
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You will require the <literal>SELECT</literal> privilege on the
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<replaceable class="parameter">data_source</replaceable> and any column(s)
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of the <replaceable class="parameter">target_table_name</replaceable>
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referred to in a <literal>condition</literal>.
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</para>
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<para>
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<command>MERGE</command> is not supported if the
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<replaceable class="parameter">target_table_name</replaceable> is a
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materialized view, foreign table, or if it has any
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rules defined on it.
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</para>
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</refsect1>
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<refsect1>
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<title>Parameters</title>
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<variablelist>
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<varlistentry>
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<term><replaceable class="parameter">target_table_name</replaceable></term>
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<listitem>
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<para>
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The name (optionally schema-qualified) of the target table to merge into.
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If <literal>ONLY</literal> is specified before the table name, matching
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rows are updated or deleted in the named table only. If
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<literal>ONLY</literal> is not specified, matching rows are also updated
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or deleted in any tables inheriting from the named table. Optionally,
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<literal>*</literal> can be specified after the table name to explicitly
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indicate that descendant tables are included. The
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<literal>ONLY</literal> keyword and <literal>*</literal> option do not
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affect insert actions, which always insert into the named table only.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">target_alias</replaceable></term>
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<listitem>
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<para>
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A substitute name for the target table. When an alias is
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provided, it completely hides the actual name of the table. For
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example, given <literal>MERGE INTO foo AS f</literal>, the remainder of the
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<command>MERGE</command> statement must refer to this table as
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<literal>f</literal> not <literal>foo</literal>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">source_table_name</replaceable></term>
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<listitem>
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<para>
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The name (optionally schema-qualified) of the source table, view, or
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transition table. If <literal>ONLY</literal> is specified before the
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table name, matching rows are included from the named table only. If
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<literal>ONLY</literal> is not specified, matching rows are also included
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from any tables inheriting from the named table. Optionally,
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<literal>*</literal> can be specified after the table name to explicitly
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indicate that descendant tables are included.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">source_query</replaceable></term>
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<listitem>
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<para>
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A query (<command>SELECT</command> statement or <command>VALUES</command>
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statement) that supplies the rows to be merged into the
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<replaceable class="parameter">target_table_name</replaceable>.
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Refer to the <xref linkend="sql-select"/>
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statement or <xref linkend="sql-values"/>
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statement for a description of the syntax.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">source_alias</replaceable></term>
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<listitem>
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<para>
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A substitute name for the data source. When an alias is
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provided, it completely hides the actual name of the table or the fact
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that a query was issued.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">join_condition</replaceable></term>
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<listitem>
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<para>
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<replaceable class="parameter">join_condition</replaceable> is
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an expression resulting in a value of type
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<type>boolean</type> (similar to a <literal>WHERE</literal>
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clause) that specifies which rows in the
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<replaceable class="parameter">data_source</replaceable>
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match rows in the
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<replaceable class="parameter">target_table_name</replaceable>.
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</para>
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<warning>
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<para>
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Only columns from <replaceable class="parameter">target_table_name</replaceable>
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that attempt to match <replaceable class="parameter">data_source</replaceable>
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rows should appear in <replaceable class="parameter">join_condition</replaceable>.
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<replaceable class="parameter">join_condition</replaceable> subexpressions that
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only reference <replaceable class="parameter">target_table_name</replaceable>
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columns can affect which action is taken, often in surprising ways.
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</para>
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</warning>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">when_clause</replaceable></term>
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<listitem>
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<para>
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At least one <literal>WHEN</literal> clause is required.
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</para>
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<para>
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If the <literal>WHEN</literal> clause specifies <literal>WHEN MATCHED</literal>
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and the candidate change row matches a row in the
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<replaceable class="parameter">target_table_name</replaceable>,
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the <literal>WHEN</literal> clause is executed if the
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<replaceable class="parameter">condition</replaceable> is
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absent or it evaluates to <literal>true</literal>.
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</para>
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<para>
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Conversely, if the <literal>WHEN</literal> clause specifies
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<literal>WHEN NOT MATCHED</literal>
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and the candidate change row does not match a row in the
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<replaceable class="parameter">target_table_name</replaceable>,
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the <literal>WHEN</literal> clause is executed if the
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<replaceable class="parameter">condition</replaceable> is
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absent or it evaluates to <literal>true</literal>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">condition</replaceable></term>
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<listitem>
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<para>
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An expression that returns a value of type <type>boolean</type>.
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If this expression for a <literal>WHEN</literal> clause
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returns <literal>true</literal>, then the action for that clause
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is executed for that row.
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</para>
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<para>
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A condition on a <literal>WHEN MATCHED</literal> clause can refer to columns
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in both the source and the target relations. A condition on a
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<literal>WHEN NOT MATCHED</literal> clause can only refer to columns from
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the source relation, since by definition there is no matching target row.
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Only the system attributes from the target table are accessible.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">merge_insert</replaceable></term>
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<listitem>
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<para>
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The specification of an <literal>INSERT</literal> action that inserts
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one row into the target table.
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The target column names can be listed in any order. If no list of
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column names is given at all, the default is all the columns of the
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table in their declared order.
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</para>
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<para>
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Each column not present in the explicit or implicit column list will be
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filled with a default value, either its declared default value
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or null if there is none.
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</para>
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<para>
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If <replaceable class="parameter">target_table_name</replaceable>
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is a partitioned table, each row is routed to the appropriate partition
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and inserted into it.
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If <replaceable class="parameter">target_table_name</replaceable>
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is a partition, an error will occur if any input row violates the
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partition constraint.
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</para>
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<para>
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Column names may not be specified more than once.
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<command>INSERT</command> actions cannot contain sub-selects.
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</para>
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<para>
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Only one <literal>VALUES</literal> clause can be specified.
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The <literal>VALUES</literal> clause can only refer to columns from
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the source relation, since by definition there is no matching target row.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">merge_update</replaceable></term>
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<listitem>
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<para>
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The specification of an <literal>UPDATE</literal> action that updates
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the current row of the <replaceable class="parameter">target_table_name</replaceable>.
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Column names may not be specified more than once.
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</para>
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<para>
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Neither a table name nor a <literal>WHERE</literal> clause are allowed.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">merge_delete</replaceable></term>
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<listitem>
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<para>
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Specifies a <literal>DELETE</literal> action that deletes the current row
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of the <replaceable class="parameter">target_table_name</replaceable>.
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Do not include the table name or any other clauses, as you would normally
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do with a <xref linkend="sql-delete"/> command.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">column_name</replaceable></term>
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<listitem>
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<para>
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The name of a column in the <replaceable
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class="parameter">target_table_name</replaceable>. The column name
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can be qualified with a subfield name or array subscript, if
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needed. (Inserting into only some fields of a composite
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column leaves the other fields null.)
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Do not include the table's name in the specification
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of a target column.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>OVERRIDING SYSTEM VALUE</literal></term>
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<listitem>
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<para>
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Without this clause, it is an error to specify an explicit value
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(other than <literal>DEFAULT</literal>) for an identity column defined
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as <literal>GENERATED ALWAYS</literal>. This clause overrides that
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restriction.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>OVERRIDING USER VALUE</literal></term>
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<listitem>
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<para>
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If this clause is specified, then any values supplied for identity
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columns defined as <literal>GENERATED BY DEFAULT</literal> are ignored
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and the default sequence-generated values are applied.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>DEFAULT VALUES</literal></term>
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<listitem>
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<para>
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All columns will be filled with their default values.
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(An <literal>OVERRIDING</literal> clause is not permitted in this
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form.)
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">expression</replaceable></term>
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<listitem>
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<para>
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An expression to assign to the column. If used in a
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<literal>WHEN MATCHED</literal> clause, the expression can use values
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from the original row in the target table, and values from the
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<literal>data_source</literal> row.
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If used in a <literal>WHEN NOT MATCHED</literal> clause, the
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expression can use values from the <literal>data_source</literal>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>DEFAULT</literal></term>
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<listitem>
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<para>
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Set the column to its default value (which will be <literal>NULL</literal>
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if no specific default expression has been assigned to it).
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">with_query</replaceable></term>
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<listitem>
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<para>
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The <literal>WITH</literal> clause allows you to specify one or more
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subqueries that can be referenced by name in the <command>MERGE</command>
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query. See <xref linkend="queries-with"/> and <xref linkend="sql-select"/>
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for details.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1>
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<title>Outputs</title>
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<para>
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On successful completion, a <command>MERGE</command> command returns a command
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tag of the form
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<screen>
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MERGE <replaceable class="parameter">total_count</replaceable>
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</screen>
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The <replaceable class="parameter">total_count</replaceable> is the total
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number of rows changed (whether inserted, updated, or deleted).
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If <replaceable class="parameter">total_count</replaceable> is 0, no rows
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were changed in any way.
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</para>
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</refsect1>
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<refsect1>
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<title>Notes</title>
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<para>
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The following steps take place during the execution of
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<command>MERGE</command>.
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<orderedlist>
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<listitem>
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<para>
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Perform any <literal>BEFORE STATEMENT</literal> triggers for all
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actions specified, whether or not their <literal>WHEN</literal>
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clauses match.
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</para>
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</listitem>
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<listitem>
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<para>
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Perform a join from source to target table.
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The resulting query will be optimized normally and will produce
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a set of candidate change rows. For each candidate change row,
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<orderedlist>
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<listitem>
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<para>
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Evaluate whether each row is <literal>MATCHED</literal> or
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<literal>NOT MATCHED</literal>.
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</para>
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</listitem>
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<listitem>
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<para>
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Test each <literal>WHEN</literal> condition in the order
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specified until one returns true.
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</para>
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</listitem>
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<listitem>
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<para>
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When a condition returns true, perform the following actions:
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<orderedlist>
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<listitem>
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<para>
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Perform any <literal>BEFORE ROW</literal> triggers that fire
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for the action's event type.
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</para>
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</listitem>
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<listitem>
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<para>
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Perform the specified action, invoking any check constraints on the
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target table.
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</para>
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</listitem>
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<listitem>
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<para>
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Perform any <literal>AFTER ROW</literal> triggers that fire for
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the action's event type.
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</para>
|
|
</listitem>
|
|
</orderedlist></para>
|
|
</listitem>
|
|
</orderedlist></para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Perform any <literal>AFTER STATEMENT</literal> triggers for actions
|
|
specified, whether or not they actually occur. This is similar to the
|
|
behavior of an <command>UPDATE</command> statement that modifies no rows.
|
|
</para>
|
|
</listitem>
|
|
</orderedlist>
|
|
In summary, statement triggers for an event type (say,
|
|
<command>INSERT</command>) will be fired whenever we
|
|
<emphasis>specify</emphasis> an action of that kind.
|
|
In contrast, row-level triggers will fire only for the specific event type
|
|
being <emphasis>executed</emphasis>.
|
|
So a <command>MERGE</command> command might fire statement triggers for both
|
|
<command>UPDATE</command> and <command>INSERT</command>, even though only
|
|
<command>UPDATE</command> row triggers were fired.
|
|
</para>
|
|
|
|
<para>
|
|
You should ensure that the join produces at most one candidate change row
|
|
for each target row. In other words, a target row shouldn't join to more
|
|
than one data source row. If it does, then only one of the candidate change
|
|
rows will be used to modify the target row; later attempts to modify the
|
|
row will cause an error.
|
|
This can also occur if row triggers make changes to the target table
|
|
and the rows so modified are then subsequently also modified by
|
|
<command>MERGE</command>.
|
|
If the repeated action is an <command>INSERT</command>, this will
|
|
cause a uniqueness violation, while a repeated <command>UPDATE</command>
|
|
or <command>DELETE</command> will cause a cardinality violation; the
|
|
latter behavior is required by the <acronym>SQL</acronym> standard.
|
|
This differs from historical <productname>PostgreSQL</productname>
|
|
behavior of joins in <command>UPDATE</command> and
|
|
<command>DELETE</command> statements where second and subsequent
|
|
attempts to modify the same row are simply ignored.
|
|
</para>
|
|
|
|
<para>
|
|
If a <literal>WHEN</literal> clause omits an <literal>AND</literal>
|
|
sub-clause, it becomes the final reachable clause of that
|
|
kind (<literal>MATCHED</literal> or <literal>NOT MATCHED</literal>).
|
|
If a later <literal>WHEN</literal> clause of that kind
|
|
is specified it would be provably unreachable and an error is raised.
|
|
If no final reachable clause is specified of either kind, it is
|
|
possible that no action will be taken for a candidate change row.
|
|
</para>
|
|
|
|
<para>
|
|
The order in which rows are generated from the data source is
|
|
indeterminate by default.
|
|
A <replaceable class="parameter">source_query</replaceable> can be
|
|
used to specify a consistent ordering, if required, which might be
|
|
needed to avoid deadlocks between concurrent transactions.
|
|
</para>
|
|
|
|
<para>
|
|
There is no <literal>RETURNING</literal> clause with
|
|
<command>MERGE</command>. Actions of <command>INSERT</command>,
|
|
<command>UPDATE</command> and <command>DELETE</command> cannot contain
|
|
<literal>RETURNING</literal> or <literal>WITH</literal> clauses.
|
|
</para>
|
|
|
|
<para>
|
|
When <command>MERGE</command> is run concurrently with other commands
|
|
that modify the target table, the usual transaction isolation rules
|
|
apply; see <xref linkend="transaction-iso"/> for an explanation
|
|
on the behavior at each isolation level.
|
|
You may also wish to consider using <command>INSERT ... ON CONFLICT</command>
|
|
as an alternative statement which offers the ability to run an
|
|
<command>UPDATE</command> if a concurrent <command>INSERT</command>
|
|
occurs. There are a variety of differences and restrictions between
|
|
the two statement types and they are not interchangeable.
|
|
</para>
|
|
</refsect1>
|
|
|
|
<refsect1>
|
|
<title>Examples</title>
|
|
|
|
<para>
|
|
Perform maintenance on <literal>customer_accounts</literal> based
|
|
upon new <literal>recent_transactions</literal>.
|
|
|
|
<programlisting>
|
|
MERGE INTO customer_account ca
|
|
USING recent_transactions t
|
|
ON t.customer_id = ca.customer_id
|
|
WHEN MATCHED THEN
|
|
UPDATE SET balance = balance + transaction_value
|
|
WHEN NOT MATCHED THEN
|
|
INSERT (customer_id, balance)
|
|
VALUES (t.customer_id, t.transaction_value);
|
|
</programlisting>
|
|
</para>
|
|
|
|
<para>
|
|
Notice that this would be exactly equivalent to the following
|
|
statement because the <literal>MATCHED</literal> result does not change
|
|
during execution.
|
|
|
|
<programlisting>
|
|
MERGE INTO customer_account ca
|
|
USING (SELECT customer_id, transaction_value FROM recent_transactions) AS t
|
|
ON t.customer_id = ca.customer_id
|
|
WHEN MATCHED THEN
|
|
UPDATE SET balance = balance + transaction_value
|
|
WHEN NOT MATCHED THEN
|
|
INSERT (customer_id, balance)
|
|
VALUES (t.customer_id, t.transaction_value);
|
|
</programlisting>
|
|
</para>
|
|
|
|
<para>
|
|
Attempt to insert a new stock item along with the quantity of stock. If
|
|
the item already exists, instead update the stock count of the existing
|
|
item. Don't allow entries that have zero stock.
|
|
<programlisting>
|
|
MERGE INTO wines w
|
|
USING wine_stock_changes s
|
|
ON s.winename = w.winename
|
|
WHEN NOT MATCHED AND s.stock_delta > 0 THEN
|
|
INSERT VALUES(s.winename, s.stock_delta)
|
|
WHEN MATCHED AND w.stock + s.stock_delta > 0 THEN
|
|
UPDATE SET stock = w.stock + s.stock_delta
|
|
WHEN MATCHED THEN
|
|
DELETE;
|
|
</programlisting>
|
|
|
|
The <literal>wine_stock_changes</literal> table might be, for example, a
|
|
temporary table recently loaded into the database.
|
|
</para>
|
|
|
|
</refsect1>
|
|
|
|
<refsect1>
|
|
<title>Compatibility</title>
|
|
<para>
|
|
This command conforms to the <acronym>SQL</acronym> standard.
|
|
</para>
|
|
<para>
|
|
The WITH clause and <literal>DO NOTHING</literal> action are extensions to
|
|
the <acronym>SQL</acronym> standard.
|
|
</para>
|
|
</refsect1>
|
|
</refentry>
|