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A.5 MySQL 8.0 FAQ: Triggers

A.5.1. Where can I find the documentation for MySQL 8.0 triggers?
A.5.2. Is there a discussion forum for MySQL Triggers?
A.5.3. Does MySQL 8.0 have statement-level or row-level triggers?
A.5.4. Are there any default triggers?
A.5.5. How are triggers managed in MySQL?
A.5.6. Is there a way to view all triggers in a given database?
A.5.7. Where are triggers stored?
A.5.8. Can a trigger call a stored procedure?
A.5.9. Can triggers access tables?
A.5.10. Can a table have multiple triggers with the same trigger event and action time?
A.5.11. Can triggers call an external application through a UDF?
A.5.12. Is it possible for a trigger to update tables on a remote server?
A.5.13. Do triggers work with replication?
A.5.14. How are actions carried out through triggers on a source replicated to a replica?

A.5.1.

Where can I find the documentation for MySQL 8.0 triggers?

See Section 24.3, “Using Triggers”.

A.5.2.

Is there a discussion forum for MySQL Triggers?

Yes. It is available at https://forums.mysql.com/list.php?99.

A.5.3.

Does MySQL 8.0 have statement-level or row-level triggers?

In MySQL 8.0, all triggers are FOR EACH ROW; that is, the trigger is activated for each row that is inserted, updated, or deleted. MySQL 8.0 does not support triggers using FOR EACH STATEMENT.

A.5.4.

Are there any default triggers?

Not explicitly. MySQL does have specific special behavior for some TIMESTAMP columns, as well as for columns which are defined using AUTO_INCREMENT.

A.5.5.

How are triggers managed in MySQL?

In MySQL 8.0, triggers can be created using the CREATE TRIGGER statement, and dropped using DROP TRIGGER. See Section 13.1.22, “CREATE TRIGGER Statement”, and Section 13.1.34, “DROP TRIGGER Statement”, for more about these statements.

Information about triggers can be obtained by querying the INFORMATION_SCHEMA.TRIGGERS table. See Section 25.45, “The INFORMATION_SCHEMA TRIGGERS Table”.

A.5.6.

Is there a way to view all triggers in a given database?

Yes. You can obtain a listing of all triggers defined on database dbname using a query on the INFORMATION_SCHEMA.TRIGGERS table such as the one shown here:

SELECT TRIGGER_NAME, EVENT_MANIPULATION, EVENT_OBJECT_TABLE, ACTION_STATEMENT
    FROM INFORMATION_SCHEMA.TRIGGERS
    WHERE TRIGGER_SCHEMA='dbname';

For more information about this table, see Section 25.45, “The INFORMATION_SCHEMA TRIGGERS Table”.

You can also use the SHOW TRIGGERS statement, which is specific to MySQL. See Section 13.7.7.40, “SHOW TRIGGERS Statement”.

A.5.7.

Where are triggers stored?

Triggers are stored in the mysql.triggers system table, which is part of the data dictionary.

A.5.8.

Can a trigger call a stored procedure?

Yes.

A.5.9.

Can triggers access tables?

A trigger can access both old and new data in its own table. A trigger can also affect other tables, but it is not permitted to modify a table that is already being used (for reading or writing) by the statement that invoked the function or trigger.

A.5.10.

Can a table have multiple triggers with the same trigger event and action time?

In MySQL 8.0, it is possible to define multiple triggers for a given table that have the same trigger event and action time. For example, you can have two BEFORE UPDATE triggers for a table. By default, triggers that have the same trigger event and action time activate in the order they were created. To affect trigger order, specify a clause after FOR EACH ROW that indicates FOLLOWS or PRECEDES and the name of an existing trigger that also has the same trigger event and action time. With FOLLOWS, the new trigger activates after the existing trigger. With PRECEDES, the new trigger activates before the existing trigger.

A.5.11.

Can triggers call an external application through a UDF?

Yes. For example, a trigger could invoke the sys_exec() UDF.

A.5.12.

Is it possible for a trigger to update tables on a remote server?

Yes. A table on a remote server could be updated using the FEDERATED storage engine. (See Section 16.8, “The FEDERATED Storage Engine”).

A.5.13.

Do triggers work with replication?

Yes. However, the way in which they work depends whether you are using MySQL's classic statement-based or row-based replication format.

When using statement-based replication, triggers on the replica are executed by statements that are executed on the source (and replicated to the replica).

When using row-based replication, triggers are not executed on the replica due to statements that were run on the source and then replicated to the replica. Instead, when using row-based replication, the changes caused by executing the trigger on the source are applied on the replica.

For more information, see Section 17.5.1.35, “Replication and Triggers”.

A.5.14.

How are actions carried out through triggers on a source replicated to a replica?

Again, this depends on whether you are using statement-based or row-based replication.

Statement-based replication.  First, the triggers that exist on a source must be re-created on the replica server. Once this is done, the replication flow works as any other standard DML statement that participates in replication. For example, consider a table EMP that has an AFTER insert trigger, which exists on a replication source server. The same EMP table and AFTER insert trigger exist on the replica server as well. The replication flow would be:

  1. An INSERT statement is made to EMP.

  2. The AFTER trigger on EMP activates.

  3. The INSERT statement is written to the binary log.

  4. The replica picks up the INSERT statement to EMP and executes it.

  5. The AFTER trigger on EMP that exists on the replica activates.

Row-based replication.  When you use row-based replication, the changes caused by executing the trigger on the source are applied on the replica. However, the triggers themselves are not actually executed on the replica under row-based replication. This is because, if both the source and the replica applied the changes from the source and, in addition, the trigger causing these changes were applied on the replica, the changes would in effect be applied twice on the replica, leading to different data on the source and the replica.

In most cases, the outcome is the same for both row-based and statement-based replication. However, if you use different triggers on the source and replica, you cannot use row-based replication. (This is because the row-based format replicates the changes made by triggers executing on the source to the replicas, rather than the statements that caused the triggers to execute, and the corresponding triggers on the replica are not executed.) Instead, any statements causing such triggers to be executed must be replicated using statement-based replication.

For more information, see Section 17.5.1.35, “Replication and Triggers”.