create_trigger.sgml

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<refentry id="SQL-CREATETRIGGER">
 <refmeta>
  <refentrytitle id="SQL-CREATETRIGGER-TITLE">
   CREATE TRIGGER
  </refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>
 <refnamediv>
  <refname>
   CREATE TRIGGER
  </refname>
  <refpurpose>
   Creates 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 of an existing trigger.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><replaceable class="parameter">table</replaceable></term>
      <listitem>
       <para>
	The name of a 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">funcname</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">relname</replaceable> and will execute
   the specified function <replaceable class="parameter">funcname</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 "visible" to the trigger.
  </para>

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

  <refsect2 id="R2-SQL-CREATETRIGGER-3">
   <refsect2info>
    <date>1998-09-21</date>
   </refsect2info>
   <title>
    Notes
   </title>
   <para>
    <command>CREATE TRIGGER</command> is a <productname>Postgres</productname>
    language extension.
   </para>
   <para>
    Only the relation owner may create a trigger on this relation.
   </para>
   <para>
    As of the current release (v7.0), STATEMENT triggers are not implemented.
   </para>
   <para>
    Refer to <command>DROP TRIGGER</command> for information on how to 
    remove triggers.
   </para>   
  </refsect2>
 </refsect1>

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

 <refsect1 id="R1-SQL-CREATETRIGGER-3">
  <title>
   Compatibility
  </title>
  
  <refsect2 id="R2-SQL-CREATETRIGGER-4">
   <refsect2info>
    <date>1998-09-21</date>
   </refsect2info>
   <title>
    SQL92
   </title>

   <para>
    There is no <command>CREATE TRIGGER</command> in <acronym>SQL92</acronym>.
   </para>

   <para>
    The second example above may 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>
  </refsect2>
 </refsect1>
</refentry>

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