create_trigger.sgml

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$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_trigger.sgml,v 1.22 2002/01/20 22:19:56 petere Exp $
PostgreSQL documentation
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<refentry id="SQL-CREATETRIGGER">
 <docinfo>
  <date>2001-09-13</date>
 </docinfo>

 <refmeta>
  <refentrytitle id="SQL-CREATETRIGGER-TITLE">CREATE TRIGGER</refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>
 <refnamediv>
  <refname>
   CREATE TRIGGER
  </refname>
  <refpurpose>
   define a new trigger
  </refpurpose>
 </refnamediv>
 <refsynopsisdiv>
  <refsynopsisdivinfo>
   <date>2000-03-25</date>
  </refsynopsisdivinfo>
  <synopsis>
CREATE TRIGGER <replaceable class="PARAMETER">name</replaceable> { BEFORE | AFTER } { <replaceable class="PARAMETER">event</replaceable> [OR ...] }
    ON <replaceable class="PARAMETER">table</replaceable> FOR EACH { ROW | STATEMENT }
    EXECUTE PROCEDURE <replaceable class="PARAMETER">func</replaceable> ( <replaceable class="PARAMETER">arguments</replaceable> )
  </synopsis>
  
  <refsect2 id="R2-SQL-CREATETRIGGER-1">
   <refsect2info>
    <date>1998-09-21</date>
   </refsect2info>
   <title>
    Inputs
   </title>
   <para>

    <variablelist>
     <varlistentry>
      <term><replaceable class="parameter">name</replaceable></term>
      <listitem>
       <para>
	The name to give the new trigger.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><replaceable class="parameter">table</replaceable></term>
      <listitem>
       <para>
	The name of an existing table.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><replaceable class="parameter">event</replaceable></term>
      <listitem>
       <para>
	One of INSERT, DELETE or UPDATE.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><replaceable class="parameter">func</replaceable></term>
      <listitem>
       <para>
	A user-supplied function.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </refsect2>

  <refsect2 id="R2-SQL-CREATETRIGGER-2">
   <refsect2info>
    <date>1998-09-21</date>
   </refsect2info>
   <title>
    Outputs
   </title>
   <para>

    <variablelist>
     <varlistentry>
      <term><computeroutput>
CREATE
       </computeroutput></term>
      <listitem>
       <para>
	This message is returned if the trigger is successfully created.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </refsect2>
 </refsynopsisdiv>
 
 <refsect1 id="R1-SQL-CREATETRIGGER-1">
  <refsect1info>
   <date>1998-09-21</date>
  </refsect1info>
  <title>
   Description
  </title>

  <para>
   <command>CREATE TRIGGER</command> will enter a new trigger into the current
   data base.  The trigger will be associated with the relation
   <replaceable class="parameter">table</replaceable> and will execute
   the specified function <replaceable class="parameter">func</replaceable>.
  </para>

  <para>
   The trigger can be specified to fire either before BEFORE the
   operation is attempted on a tuple (before constraints are checked and
   the <command>INSERT</command>, <command>UPDATE</command> or
   <command>DELETE</command> is attempted) or AFTER the operation has
   been attempted (e.g., after constraints are checked and the
   <command>INSERT</command>, <command>UPDATE</command> or
   <command>DELETE</command> has completed). If the trigger fires before
   the event, the trigger may skip the operation for the current tuple,
   or change the tuple being inserted (for <command>INSERT</command> and
   <command>UPDATE</command> operations only). If the trigger fires
   after the event, all changes, including the last insertion, update,
   or deletion, are <quote>visible</quote> to the trigger.
  </para>
  <para>
  <command>SELECT</command> does not modify any rows so you can not
  create <command>SELECT</command> triggers. Rules and views are more
  appropriate in such cases.
  </para>

  <para>
   Refer to the chapters on SPI and Triggers in the
   <citetitle>PostgreSQL Programmer's Guide</citetitle>  for  more
   information.
  </para>
 </refsect1>

 <refsect1 id="SQL-CREATETRIGGER-notes">
  <title>Notes</title>

  <para>
   To create a trigger on a table, the user must have the
   <literal>TRIGGER</literal> privilege on the table.
  </para>

  <para>
   As of the current release, <literal>STATEMENT</literal> triggers are not implemented.
  </para>

  <para>
   Refer to the <xref linkend="sql-droptrigger"> command for
   information on how to remove triggers.
  </para>
 </refsect1>

 <refsect1 id="R1-SQL-CREATETRIGGER-2">
  <title>Examples</title>

  <para>
   Check if the specified distributor code exists in the distributors
   table before appending or updating a row in the table films:

<programlisting>
CREATE TRIGGER if_dist_exists
    BEFORE INSERT OR UPDATE ON films FOR EACH ROW
    EXECUTE PROCEDURE check_primary_key ('did', 'distributors', 'did');
</programlisting>
  </para>

  <para>
   Before cancelling a distributor or updating its code, remove every
   reference to the table films:
<programlisting>
CREATE TRIGGER if_film_exists 
    BEFORE DELETE OR UPDATE ON distributors FOR EACH ROW
    EXECUTE PROCEDURE check_foreign_key (1, 'CASCADE', 'did', 'films', 'did');
</programlisting>
  </para>

  <para>
   The second example can also be done by using a foreign key,
   constraint as in:

<programlisting>
CREATE TABLE distributors (
    did      DECIMAL(3),
    name     VARCHAR(40),
    CONSTRAINT if_film_exists
    FOREIGN KEY(did) REFERENCES films
    ON UPDATE CASCADE ON DELETE CASCADE  
);
</programlisting>
  </para>
 </refsect1>

 <refsect1 id="SQL-CREATETRIGGER-compatibility">
  <title>Compatibility</title>
  
  <variablelist>
   <varlistentry>
    <term>SQL92</term>
    <listitem>
     <para>
      There is no <command>CREATE TRIGGER</command> statement in <acronym>SQL92</acronym>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>SQL99</term>
    <listitem>
     <para>
      The <command>CREATE TRIGGER</command> statement in
      <productname>PostgreSQL</productname> implements a subset of the
      SQL99 standard.  The following functionality is missing:
      <itemizedlist>
       <listitem>
        <para>
         SQL99 allows triggers to fire on updates to specific columns
         (e.g., <literal>AFTER UPDATE OF col1, col2</literal>).
        </para>
       </listitem>

       <listitem>
        <para>
         SQL99 allows you to define aliases for the <quote>old</quote>
         and <quote>new</quote> rows or tables for use in the definition
         of the triggered action (e.g., <literal>CREATE TRIGGER ... ON
         tablename REFERENCING OLD ROW AS somename NEW ROW AS
         othername ...</literal>).  Since
         <productname>PostgreSQL</productname> allows trigger
         procedures to be written in any number of user-defined
         languages, access to the data is handled in a
         language-specific way.
        </para>
       </listitem>

       <listitem>
        <para>
         <productname>PostgreSQL</productname> only has row-level
         triggers, no statement-level triggers.
        </para>
       </listitem>

       <listitem>
        <para>
         <productname>PostgreSQL</productname> only allows the
         execution of a stored procedure for the triggered action.
         SQL99 allows the execution of a number of other SQL commands,
         such as <command>CREATE TABLE</command> as triggered action.
         This limitation is not hard to work around by creating a
         stored procedure that executes these commands.
        </para>
       </listitem>
      </itemizedlist>
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1>
  <title>See Also</title>

  <simplelist type="inline">
   <member><xref linkend="sql-createfunction"></member>
   <member><xref linkend="sql-droptrigger"></member>
   <member><citetitle>PostgreSQL Programmer's Guide</citetitle></member>
  </simplelist>
 </refsect1>
</refentry>

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