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11.16.1.The CREATE TRIGGER statement

Triggers can be defined to act upon a table or column and fire upon:

UPDATE
INSERT
DELETE

at the following times during the operation on a table or column:

BEFORE
AFTER
INSTEAD OF

Triggers have a unique name which is qualified by the current catalog and owner. The trigger name is only really relevant for the purposes of dropping triggers. Triggers operate on a table or column which must be adequately qualified.

The trigger body has read-only access to the values of the data manipulation operation that triggered the trigger. In the case of an update statement it has access to both old and new values for each effected column. These values cannot be changed directly. If the trigger is to influence any data in a table, even from the current operation, it must be achieved by another SQL statement. The REFERENCING clause allows specifying a correlation name for new and old values of columns. By default, the new values are seen under the column names without a correlation name. If old values of updated columns are needed, the REFERENCING OLD AS <alias> will make <alias>.<column> refer to the old value.

Triggers defined to make further operations within the same or other table may fire further triggers, or even the same trigger again. Care must be taken to understand the implications of this and when triggers cane be allowed to continue firing after the current trigger. For example, an after update trigger that makes a further update to the same table will fire the same trigger again and may continue looping in this way endlessly. The SET TRIGGER statement can be issued to control this:

SET TRIGGERS on; -- (default state) further triggers within this transaction are allowed to fire.
SET TRIGGERS off; -- further triggers within this transaction are disabled.

A table may have more than one trigger. Their execution order can be specified using the ORDER clause. Each trigger gets an order number, triggers are called starting at the lowest order number in ascending order.

Syntax:

CREATE TRIGGER NAME action_time event ON q_table_name
  opt_order opt_old_ref trig_action

action_time
        : BEFORE
        | AFTER

event
        : INSERT
        | UPDATE opt_column_commalist
        | DELETE

opt_order
        | ORDER INTNUM

opt_old_ref
        | REFERENCING old_commalist

trig_action
        : compound_statement

old_commalist
        : old_alias
        | old_commalist ',' old_alias

old_alias
        : OLD AS NAME
        | NEW AS NAME

Example11.8.Creating a simple trigger

This trigger is a simple example of one that would cause an endless loop if further triggering were not disabled.

create trigger update_mydate after
   update on mytable referencing old as O, new as N
{
  set triggers off;
  update mytable
    set
      previousdate = O.mydate,
      mydate=now()
    where id=N.id;
}
;

The trigger makes aliases for the values of the column that are part of the SQL manipulation transaction that will be in progress, hence the values of the columns can be accessed as "O.column" and "N.column" for old and new values respectively.

The set statement is scope to the procedure or trigger body where it occurs, plus procedures called from there , thus when the trigger finishes no other triggers are effected by it.


Example11.9.Creating a simple trigger using INSTEAD OF

This trigger example will show how INSTEAD OF can be used to intercept the values of an insert statement and re-write it. In this case the purpose is to deliberately truncate VARCHAR inserts to prevent an error if the data type bounds are exceeded:

First we create a test table with a 30 character limitation in one of the columns:

SQL>create table test_trunc (
      id integer not null primary key,
      txt varchar (30)
      )
    ;

Done. -- 10 msec.

Then we attempt to insert 33 characters into it with the following results:

SQL>insert into test_trunc (id, txt)
      values (1, 'aaaaaaaaaabbbbbbbbbbccccccccccxxx');

*** Error 22026: [Virtuoso ODBC Driver][Virtuoso Server]SR319: Max column length (30) of column [txt] exceeded

Now we make a trigger to fire instead of insert statements that can perform some custom error correction, in this case we simply want to chop-off any extra characters that will cause an insert to fail.

SQL>create trigger test_trunc_it
      instead of insert on test_trunc
        referencing new as N
    {
      set triggers off; -- we do not want this looping...
      insert into test_trunc (id, txt) values (N.id, left(N.txt, 30));
    }
    ;

Done. -- 10 msec.

We perform the same test insert, now without errors:

SQL>insert into test_trunc (id, txt)
  values (1, 'aaaaaaaaaabbbbbbbbbbccccccccccxxx');

Done. -- 10 msec.

And to see what we have in the database, a quick select:

SQL> select * from test_trunc;
id                txt
INTEGER NOT NULL  VARCHAR
______________________________________________________

1                 aaaaaaaaaabbbbbbbbbbcccccccccc

1 Rows. -- 20 msec.

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