Microsoft® SQL Server™ 2000 triggers are a special class of stored procedure defined to execute automatically when an UPDATE, INSERT, or DELETE statement is issued against a table or view. Triggers are powerful tools that sites can use to enforce their business rules automatically when data is modified. Triggers can extend the integrity checking logic of SQL Server constraints, defaults, and rules, although constraints and defaults should be used instead whenever they provide all the needed functionality.
Tables can have multiple triggers. The CREATE TRIGGER statement can be defined with the FOR UPDATE, FOR INSERT, or FOR DELETE clauses to target a trigger to a specific class of data modification actions. When FOR UPDATE is specified, the IF UPDATE (column_name) clause can be used to target a trigger to updates affecting a particular column.
Triggers can automate the processing for a company. In an inventory system, update triggers can detect when a stock level reaches a reorder point and generate an order to the supplier automatically. In a database recording the processes in a factory, triggers can e-mail or page operators when a process exceeds defined safety limits.
The following trigger generates an e-mail whenever a new title is added in the pubs database:
CREATE TRIGGER reminder ON titles FOR INSERT AS
EXEC master..xp_sendmail 'MaryM',
'New title, mention in the next report to distributors.'
Triggers contain Transact-SQL statements, much the same as stored procedures. Triggers, like stored procedures, return the result set generated by any SELECT statements in the trigger. Including SELECT statements in triggers, except statements that only fill parameters, is not recommended. This is because users do not expect to see any result sets returned by an UPDATE, INSERT, or DELETE statement.
You can use the FOR clause to specify when a trigger is executed:
The trigger executes after the statement that triggered it completes. If the statement fails with an error, such as a constraint violation or syntax error, the trigger is not executed. AFTER triggers cannot be specified for views, they can only be specified for tables. You can specify multiple AFTER triggers for each triggering action (INSERT, UPDATE, or DELETE). If you have multiple AFTER triggers for a table, you can use sp_settriggerorder to define which AFTER trigger fires first and which fires last. All other AFTER triggers besides the first and last fire in an undefined order which you cannot control.
AFTER is the default in SQL Server 2000. You could not specify AFTER or INSTEAD OF in SQL Server version 7.0 or earlier, all triggers in those versions operated as AFTER triggers.
- INSTEAD OF
The trigger executes in place of the triggering action. INSTEAD OF triggers can be specified on both tables and views. You can define only one INSTEAD OF trigger for each triggering action (INSERT, UPDATE, and DELETE). INSTEAD OF triggers can be used to perform enhance integrity checks on the data values supplied in INSERT and UPDATE statements. INSTEAD OF triggers also let you specify actions that allow views, which would normally not support updates, to be updatable.