B.5 MySQL 5.5 FAQ: Triggers

Questions

Questions and Answers

B.5.1: Where can I find the documentation for MySQL 5.5 triggers?

See Section 19.3, “Using Triggers”.

B.5.2: Is there a discussion forum for MySQL Triggers?

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

B.5.3: Does MySQL 5.5 have statement-level or row-level triggers?

In MySQL 5.5, all triggers are FOR EACH ROW—that is, the trigger is activated for each row that is inserted, updated, or deleted. MySQL 5.5 does not support triggers using FOR EACH STATEMENT.

B.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.

B.5.5: How are triggers managed in MySQL?

In MySQL 5.5, triggers can be created using the CREATE TRIGGER statement, and dropped using DROP TRIGGER. See Section 13.1.19, “CREATE TRIGGER Syntax”, and Section 13.1.30, “DROP TRIGGER Syntax”, for more about these statements.

Information about triggers can be obtained by querying the INFORMATION_SCHEMA.TRIGGERS table. See Section 20.25, “The INFORMATION_SCHEMA TRIGGERS Table”.

B.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 20.25, “The INFORMATION_SCHEMA TRIGGERS Table”.

You can also use the SHOW TRIGGERS statement, which is specific to MySQL. See Section 13.7.5.39, “SHOW TRIGGERS Syntax”.

B.5.7: Where are triggers stored?

Triggers for a table are currently stored in .TRG files, with one such file one per table.

B.5.8: Can a trigger call a stored procedure?

Yes.

B.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.

B.5.10: Can a table have multiple triggers with the same trigger event and action time?

In MySQL 5.5, there cannot be multiple triggers for a given table that have the same trigger event and action time. For example, you cannot have two BEFORE UPDATE triggers for a table. This limitation is lifted in MySQL 5.7.

B.5.11: Can triggers call an external application through a UDF?

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

B.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 14.10, “The FEDERATED Storage Engine”).

B.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 replication available in all versions of MySQL, or the row-based replication format introduced in MySQL 5.1.

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

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

For more information, see Section 16.4.1.34, “Replication and Triggers”.

B.5.14: How are actions carried out through triggers on a master replicated to a slave?

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

Statement-based replication.  First, the triggers that exist on a master must be re-created on the slave 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 master MySQL server. The same EMP table and AFTER insert trigger exist on the slave 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 replication slave picks up the INSERT statement to EMP and executes it.

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

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

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

For more information, see Section 16.4.1.34, “Replication and Triggers”.