Creating a trigger on a table (SQL)

Create a trigger on a table by using the CREATE TRIGGER statement. The body of a trigger consists of a compound statement: a set of semicolon-delimited SQL statements bracketed by a BEGIN and an END statement.

Prérequis

You must have the CREATE ANY TRIGGER or CREATE ANY OBJECT system privilege. Additionally, you must be the owner of the table the trigger is built on or have one of the following privileges:

ALTER privilege on the table
ALTER ANY TABLE system privilege
ALTER ANY OBJECT system privilege

Contexte et remarques

You cannot use COMMIT and ROLLBACK and some ROLLBACK TO SAVEPOINT statements within a trigger.

Task

Connect to the database.
Execute a CREATE TRIGGER statement.

Résultat

The new trigger is created.

Exemple

Example 1: A row-level INSERT trigger The following trigger is an example of a row-level INSERT trigger. It checks that the birth date entered for a new employee is reasonable:

CREATE TRIGGER check_birth_date AFTER INSERT ON Employees REFERENCING NEW AS new_employee FOR EACH ROW BEGIN DECLARE err_user_error EXCEPTION FOR SQLSTATE '99999'; IF new_employee.BirthDate > 'June 6, 2001' THEN SIGNAL err_user_error; END IF; END;

Note

You may already have a trigger with the name check_birth_date in your SQL Anywhere sample database. If so, and you attempt to run the above SQL statement, an error is returned indicating that the trigger definition conflicts with existing triggers.

This trigger fires after any row is inserted into the Employees table. It detects and disallows any new rows that correspond to birth dates later than June 6, 2001.

The phrase REFERENCING NEW AS new_employee allows statements in the trigger code to refer to the data in the new row using the alias new_employee.

Signaling an error causes the triggering statement, and any previous trigger effects, to be undone.

For an INSERT statement that adds many rows to the Employees table, the check_birth_date trigger fires once for each new row. If the trigger fails for any of the rows, all effects of the INSERT statement roll back.

You can specify that the trigger fires before the row is inserted, rather than after, by changing the second line of the example to say

BEFORE INSERT ON Employees

The REFERENCING NEW clause refers to the inserted values of the row; it is independent of the timing (BEFORE or AFTER) of the trigger.

Sometimes it is easier to enforce constraints using declarative referential integrity or CHECK constraints, rather than triggers. For example, implementing the above example with a column check constraint proves more efficient and concise:

CHECK (@col <= 'June 6, 2001')

Example 2: A row-level DELETE trigger example The following CREATE TRIGGER statement defines a row-level DELETE trigger:

CREATE TRIGGER mytrigger BEFORE DELETE ON Employees REFERENCING OLD AS oldtable FOR EACH ROW BEGIN ... END;

The REFERENCING OLD clause is independent of the timing (BEFORE or AFTER) of the trigger, and enables the delete trigger code to refer to the values in the row being deleted using the alias oldtable.

Example 3: A statement-level UPDATE trigger example The following CREATE TRIGGER statement is appropriate for statement-level UPDATE triggers:

CREATE TRIGGER mytrigger AFTER UPDATE ON Employees REFERENCING NEW AS table_after_update OLD AS table_before_update FOR EACH STATEMENT BEGIN ... END;

The REFERENCING NEW and REFERENCING OLD clause allows the UPDATE trigger code to refer to both the old and new values of the rows being updated. The table alias table_after_update refers to columns in the new row and the table alias table_before_update refers to columns in the old row.

The REFERENCING NEW and REFERENCING OLD clause has a slightly different meaning for statement-level and row-level triggers. For statement-level triggers the REFERENCING OLD or NEW aliases are table aliases, while in row-level triggers they refer to the row being altered.