Six base options are available for the event-phase combination for tables and views:

These base forms are for creating single phase/single-event triggers. Firebird also supports forms for creating triggers for one phase and multiple-events, BEFORE INSERT OR UPDATE OR DELETE, for example, or AFTER UPDATE OR DELETE: the combinations are your choice.

The Boolean context variables INSERTING, UPDATING and DELETING can be used in the body of a trigger to determine the type of event that fired the trigger.

For DML triggers, the Firebird engine provides access to sets of OLD and NEW context variables. Each is an array of the values of the entire row: one for the values as they are before the data-changing event (the BEFORE phase) and one for the values as they will be after the event (the AFTER phase). They are referenced in statements using the form NEW.column_name and OLD.column_name, respectively. The column_name can be any column in the table’s definition, not just those that are being updated.

The NEW and OLD variables are subject to some rules:

• In all triggers, the OLD value is read-only

• In BEFORE UPDATE and BEFORE INSERT code, the NEW value is read/write, unless it is a COMPUTED BY column

• In INSERT triggers, references to the OLD variables are invalid and will throw an exception

• In DELETE triggers, references to the NEW variables are invalid and will throw an exception

• In all AFTER trigger code, the NEW variables are read-only

1. BEFORE triggers are fired before changes to the system tables. AFTER triggers are fired after system table changes.

   ☝

   Important Rule

   The event type [BEFORE | AFTER] of a DDL trigger cannot be changed.

2. When a DDL statement fires a trigger that raises an exception (BEFORE or AFTER, intentionally or unintentionally) the statement will not be committed. That is, exceptions can be used to ensure that a DDL operation will fail if the conditions are not precisely as intended.

3. DDL trigger actions are executed only when committing the transaction in which the affected DDL command runs. Never overlook the fact that what is possible to do in an AFTER trigger is exactly what is possible to do after a DDL command without autocommit. You cannot, for example, create a table and then use it in the trigger.

4. With CREATE OR ALTER statements, a trigger is fired one time at the CREATE event or the ALTER event, according to the previous existence of the object. With RECREATE statements, a trigger is fired for the DROP event if the object exists, and for the CREATE event.

5. ALTER and DROP events are generally not fired when the object name does not exist. For the exception, see point 6.

6. The exception to rule 5 is that BEFORE ALTER/DROP USER triggers fire even when the username does not exist. This is because, underneath, these commands perform DML on the security database, and the verification is not done before the command on it is run. This is likely to be different with embedded users, so do not write code that depends on this.

7. If some exception is raised after the DDL command starts its execution and before AFTER triggers are fired, AFTER triggers will not be fired.

8. Packaged procedures and triggers do not fire individual {CREATE | ALTER | DROP} {PROCEDURE | FUNCTION} triggers.

When a DDL trigger is running, the DDL_TRIGGER namespace is available for use with RDB$GET_CONTEXT. This namespace contains information on the currently firing trigger.

See also The DDL_TRIGGER Namespace in RDB$GET_CONTEXT in Chapter Built-in Scalar Functions.