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$PostgreSQL: pgsql/doc/src/sgml/ref/update.sgml,v 1.40 2006/09/03 22:37:05 tgl Exp $
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<refentry id="SQL-UPDATE">
 <refmeta>
  <refentrytitle id="SQL-UPDATE-TITLE">UPDATE</refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>UPDATE</refname>
  <refpurpose>update rows of a table</refpurpose>
 </refnamediv>

 <indexterm zone="sql-update">
  <primary>UPDATE</primary>
 </indexterm>

 <refsynopsisdiv>
<synopsis>
UPDATE [ ONLY ] <replaceable class="PARAMETER">table</replaceable> [ [ AS ] <replaceable class="parameter">alias</replaceable> ]
    SET { <replaceable class="PARAMETER">column</replaceable> = { <replaceable class="PARAMETER">expression</replaceable> | DEFAULT } |
          ( <replaceable class="PARAMETER">column</replaceable> [, ...] ) = ( { <replaceable class="PARAMETER">expression</replaceable> | DEFAULT } [, ...] ) } [, ...]
    [ FROM <replaceable class="PARAMETER">fromlist</replaceable> ]
    [ WHERE <replaceable class="PARAMETER">condition</replaceable> ]
    [ RETURNING * | <replaceable class="parameter">output_expression</replaceable> [ AS <replaceable class="parameter">output_name</replaceable> ] [, ...] ]
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>UPDATE</command> changes the values of the specified
   columns in all rows that satisfy the condition. Only the columns to
   be modified need be mentioned in the <literal>SET</literal> clause;
   columns not explicitly modified retain their previous values.
  </para>

  <para>
   By default, <command>UPDATE</command> will update rows in the
   specified table and all its subtables. If you wish to only update
   the specific table mentioned, you must use the <literal>ONLY</>
   clause.
  </para>

  <para>
   There are two ways to modify a table using information contained in
   other tables in the database: using sub-selects, or specifying
   additional tables in the <literal>FROM</literal> clause. Which
   technique is more appropriate depends on the specific
   circumstances.
  </para>

  <para>
   The optional <literal>RETURNING</> clause causes <command>UPDATE</>
   to compute and return value(s) based on each row actually updated.
   Any expression using the table's columns, and/or columns of other
   tables mentioned in <literal>FROM</literal>, can be computed.
   The new (post-update) values of the table's columns are used.
   The syntax of the <literal>RETURNING</> list is identical to that of the
   output list of <command>SELECT</>.
  </para>

  <para>
   You must have the <literal>UPDATE</literal> privilege on the table
   to update it, as well as the <literal>SELECT</literal>
   privilege to any table whose values are read in the
   <replaceable class="parameter">expressions</replaceable> or
   <replaceable class="parameter">condition</replaceable>.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>
   <varlistentry>
    <term><replaceable class="PARAMETER">table</replaceable></term>
    <listitem>
     <para>
      The name (optionally schema-qualified) of the table to update.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">alias</replaceable></term>
    <listitem>
     <para>
      A substitute name for the target table. When an alias is
      provided, it completely hides the actual name of the table.  For
      example, given <literal>UPDATE foo AS f</>, the remainder of the
      <command>UPDATE</command> statement must refer to this table as
      <literal>f</> not <literal>foo</>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="PARAMETER">column</replaceable></term>
    <listitem>
     <para>
      The name of a column in <replaceable
      class="PARAMETER">table</replaceable>.
      The column name can be qualified with a subfield name or array
      subscript, if needed.  Do not include the table's name in the
      specification of a target column &mdash; for example,
      <literal>UPDATE tab SET tab.col = 1</> is invalid.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="PARAMETER">expression</replaceable></term>
    <listitem>
     <para>
      An expression to assign to the column.  The expression may use the
      old values of this and other columns in the table.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>DEFAULT</literal></term>
    <listitem>
     <para>
      Set the column to its default value (which will be NULL if no
      specific default expression has been assigned to it).
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="PARAMETER">fromlist</replaceable></term>
    <listitem>
     <para>
      A list of table expressions, allowing columns from other tables
      to appear in the <literal>WHERE</> condition and the update
      expressions. This is similar to the list of tables that can be
      specified in the <xref linkend="sql-from"
      endterm="sql-from-title"> of a <command>SELECT</command>
      statement.  Note that the target table must not appear in the
      <replaceable>fromlist</>, unless you intend a self-join (in which
      case it must appear with an alias in the <replaceable>fromlist</>).
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="PARAMETER">condition</replaceable></term>
    <listitem>
     <para>
      An expression that returns a value of type <type>boolean</type>.
      Only rows for which this expression returns <literal>true</>
      will be updated.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="PARAMETER">output_expression</replaceable></term>
    <listitem>
     <para>
      An expression to be computed and returned by the <command>UPDATE</>
      command after each row is updated.  The expression may use any
      column names of the <replaceable class="PARAMETER">table</replaceable>
      or table(s) listed in <literal>FROM</>.
      Write <literal>*</> to return all columns.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="PARAMETER">output_name</replaceable></term>
    <listitem>
     <para>
      A name to use for a returned column.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1>
  <title>Outputs</title>

  <para>
   On successful completion, an <command>UPDATE</> command returns a command
   tag of the form
<screen>
UPDATE <replaceable class="parameter">count</replaceable>
</screen>
   The <replaceable class="parameter">count</replaceable> is the number
   of rows updated.  If <replaceable class="parameter">count</replaceable> is
   0, no rows matched the <replaceable
   class="parameter">condition</replaceable> (this is not considered
   an error).
  </para>

  <para>
   If the <command>UPDATE</> command contains a <literal>RETURNING</>
   clause, the result will be similar to that of a <command>SELECT</>
   statement containing the columns and values defined in the
   <literal>RETURNING</> list, computed over the row(s) updated by the
   command.
  </para>
 </refsect1>

 <refsect1>
  <title>Notes</title>

  <para>
   When a <literal>FROM</> clause is present, what essentially happens
   is that the target table is joined to the tables mentioned in the
   <replaceable>fromlist</replaceable>, and each output row of the join
   represents an update operation for the target table.  When using
   <literal>FROM</> you should ensure that the join
   produces at most one output row for each row to be modified.  In
   other words, a target row shouldn't join to more than one row from
   the other table(s).  If it does, then only one of the join rows
   will be used to update the target row, but which one will be used
   is not readily predictable.
  </para>

  <para>
   Because of this indeterminacy, referencing other tables only within
   sub-selects is safer, though often harder to read and slower than
   using a join.
  </para>
 </refsect1>

 <refsect1>
  <title>Examples</title>

  <para>
   Change the word <literal>Drama</> to <literal>Dramatic</> in the
   column <structfield>kind</> of the table <structname>films</structname>:

<programlisting>
UPDATE films SET kind = 'Dramatic' WHERE kind = 'Drama';
</programlisting>
  </para>

  <para>
   Adjust temperature entries and reset precipitation to its default
   value in one row of the table <structname>weather</structname>:

<programlisting>
UPDATE weather SET temp_lo = temp_lo+1, temp_hi = temp_lo+15, prcp = DEFAULT
  WHERE city = 'San Francisco' AND date = '2003-07-03';
</programlisting>
  </para>

  <para>
   Perform the same operation and return the updated entries:

<programlisting>
UPDATE weather SET temp_lo = temp_lo+1, temp_hi = temp_lo+15, prcp = DEFAULT
  WHERE city = 'San Francisco' AND date = '2003-07-03'
  RETURNING temp_lo, temp_hi, prcp;
</programlisting>
  </para>

  <para>
   Use the alternative column-list syntax to do the same update:
<programlisting>
UPDATE weather SET (temp_lo, temp_hi, prcp) = (temp_lo+1, temp_lo+15, DEFAULT)
  WHERE city = 'San Francisco' AND date = '2003-07-03';
</programlisting>
  </para>

  <para>
   Increment the sales count of the salesperson who manages the
   account for Acme Corporation, using the <literal>FROM</literal>
   clause syntax:
<programlisting>
UPDATE employees SET sales_count = sales_count + 1 FROM accounts
  WHERE accounts.name = 'Acme Corporation'
  AND employees.id = accounts.sales_person;
</programlisting>
  </para>

  <para>
   Perform the same operation, using a sub-select in the
   <literal>WHERE</literal> clause:
<programlisting>
UPDATE employees SET sales_count = sales_count + 1 WHERE id =
  (SELECT sales_person FROM accounts WHERE name = 'Acme Corporation');
</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. To do this without failing the entire transaction, use savepoints.
<programlisting>
BEGIN;
-- other operations
SAVEPOINT sp1;
INSERT INTO wines VALUES('Chateau Lafite 2003', '24');
-- Assume the above fails because of a unique key violation,
-- so now we issue these commands:
ROLLBACK TO sp1;
UPDATE wines SET stock = stock + 24 WHERE winename = 'Chateau Lafite 2003';
-- continue with other operations, and eventually
COMMIT;
</programlisting>
  </para>
 </refsect1>

 <refsect1>
  <title>Compatibility</title>

  <para>
   This command conforms to the <acronym>SQL</acronym> standard, except
   that the <literal>FROM</literal> and <literal>RETURNING</> clauses
   are <productname>PostgreSQL</productname> extensions.
  </para>

  <para>
   According to the standard, the column-list syntax should allow a list
   of columns to be assigned from a single row-valued expression, such
   as a sub-select:
<programlisting>
UPDATE accounts SET (contact_last_name, contact_first_name) =
    (SELECT last_name, first_name FROM salesmen
     WHERE salesmen.id = accounts.sales_id);
</programlisting>
   This is not currently implemented &mdash; the source must be a list
   of independent expressions.
  </para>

  <para>
   Some other database systems offer a <literal>FROM</> option in which
   the target table is supposed to be listed again within <literal>FROM</>.
   That is not how <productname>PostgreSQL</productname> interprets
   <literal>FROM</>.  Be careful when porting applications that use this
   extension.
  </para>
 </refsect1>
</refentry>

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