23.3.1 Trigger Syntax and Examples
Here is a simple example that associates a trigger with a table, to activate for
INSERT operations. The trigger acts as an accumulator, summing the values inserted into one of the columns of the table.
mysql> CREATE TABLE account (acct_num INT, amount DECIMAL(10,2)); Query OK, 0 rows affected (0.03 sec) mysql> CREATE TRIGGER ins_sum BEFORE INSERT ON account FOR EACH ROW SET @sum = @sum + NEW.amount; Query OK, 0 rows affected (0.01 sec)
CREATE TRIGGER statement creates a trigger named
ins_sum that is associated with the
account table. It also includes clauses that specify the trigger action time, the triggering event, and what to do when the trigger activates:
BEFOREindicates the trigger action time. In this case, the trigger activates before each row inserted into the table. The other permitted keyword here is
INSERTindicates the trigger event; that is, the type of operation that activates the trigger. In the example,
INSERToperations cause trigger activation. You can also create triggers for
The statement following
FOR EACH ROWdefines the trigger body; that is, the statement to execute each time the trigger activates, which occurs once for each row affected by the triggering event. In the example, the trigger body is a simple
SETthat accumulates into a user variable the values inserted into the
amountcolumn. The statement refers to the column as
NEW.amountwhich means “the value of the
amountcolumn to be inserted into the new row.”
To use the trigger, set the accumulator variable to zero, execute an
INSERT statement, and then see what value the variable has afterward:
mysql> SET @sum = 0; mysql> INSERT INTO account VALUES(137,14.98),(141,1937.50),(97,-100.00); mysql> SELECT @sum AS 'Total amount inserted'; +-----------------------+ | Total amount inserted | +-----------------------+ | 1852.48 | +-----------------------+
In this case, the value of
@sum after the
INSERT statement has executed is
14.98 + 1937.50 - 100, or
To destroy the trigger, use a
DROP TRIGGER statement. You must specify the schema name if the trigger is not in the default schema:
mysql> DROP TRIGGER test.ins_sum;
If you drop a table, any triggers for the table are also dropped.
Trigger names exist in the schema namespace, meaning that all triggers must have unique names within a schema. Triggers in different schemas can have the same name.
As of MySQL 5.7.2, 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
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.
For example, the following trigger definition defines another
BEFORE INSERT trigger for the
mysql> CREATE TRIGGER ins_transaction BEFORE INSERT ON account FOR EACH ROW PRECEDES ins_sum SET @deposits = @deposits + IF(NEW.amount>0,NEW.amount,0), @withdrawals = @withdrawals + IF(NEW.amount<0,-NEW.amount,0); Query OK, 0 rows affected (0.01 sec)
ins_transaction, is similar to
ins_sum but accumulates deposits and withdrawals separately. It has a
PRECEDES clause that causes it to activate before
ins_sum; without that clause, it would activate after
ins_sum because it is created after
Before MySQL 5.7.2, 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. To work around this, you can define a trigger that executes multiple statements by using the
BEGIN ... END compound statement construct after
FOR EACH ROW. (An example appears later in this section.)
Within the trigger body, the
NEW keywords enable you to access columns in the rows affected by a trigger.
NEW are MySQL extensions to triggers; they are not case-sensitive.
INSERT trigger, only
NEW. can be used; there is no old row. In a
DELETE trigger, only
OLD. can be used; there is no new row. In an
UPDATE trigger, you can use
OLD. to refer to the columns of a row before it is updated and
NEW. to refer to the columns of the row after it is updated.
A column named with
OLD is read only. You can refer to it (if you have the
SELECT privilege), but not modify it. You can refer to a column named with
NEW if you have the
SELECT privilege for it. In a
BEFORE trigger, you can also change its value with
SET NEW. if you have the
UPDATE privilege for it. This means you can use a trigger to modify the values to be inserted into a new row or used to update a row. (Such a
SET statement has no effect in an
AFTER trigger because the row change will have already occurred.)
BEFORE trigger, the
NEW value for an
AUTO_INCREMENT column is 0, not the sequence number that is generated automatically when the new row actually is inserted.
By using the
BEGIN ... END construct, you can define a trigger that executes multiple statements. Within the
BEGIN block, you also can use other syntax that is permitted within stored routines such as conditionals and loops. However, just as for stored routines, if you use the mysql program to define a trigger that executes multiple statements, it is necessary to redefine the mysql statement delimiter so that you can use the
; statement delimiter within the trigger definition. The following example illustrates these points. It defines an
UPDATE trigger that checks the new value to be used for updating each row, and modifies the value to be within the range from 0 to 100. This must be a
BEFORE trigger because the value must be checked before it is used to update the row:
mysql> delimiter // mysql> CREATE TRIGGER upd_check BEFORE UPDATE ON account FOR EACH ROW BEGIN IF NEW.amount < 0 THEN SET NEW.amount = 0; ELSEIF NEW.amount > 100 THEN SET NEW.amount = 100; END IF; END;// mysql> delimiter ;
It can be easier to define a stored procedure separately and then invoke it from the trigger using a simple
CALL statement. This is also advantageous if you want to execute the same code from within several triggers.
There are limitations on what can appear in statements that a trigger executes when activated:
The trigger cannot use the
CALLstatement to invoke stored procedures that return data to the client or that use dynamic SQL. (Stored procedures are permitted to return data to the trigger through
The trigger cannot use statements that explicitly or implicitly begin or end a transaction, such as
ROLLBACK to SAVEPOINTis permitted because it does not end a transaction.).
MySQL handles errors during trigger execution as follows:
BEFOREtrigger fails, the operation on the corresponding row is not performed.
BEFOREtrigger is activated by the attempt to insert or modify the row, regardless of whether the attempt subsequently succeeds.
AFTERtrigger is executed only if any
BEFOREtriggers and the row operation execute successfully.
An error during either a
AFTERtrigger results in failure of the entire statement that caused trigger invocation.
For transactional tables, failure of a statement should cause rollback of all changes performed by the statement. Failure of a trigger causes the statement to fail, so trigger failure also causes rollback. For nontransactional tables, such rollback cannot be done, so although the statement fails, any changes performed prior to the point of the error remain in effect.
Triggers can contain direct references to tables by name, such as the trigger named
testref shown in this example:
CREATE TABLE test1(a1 INT); CREATE TABLE test2(a2 INT); CREATE TABLE test3(a3 INT NOT NULL AUTO_INCREMENT PRIMARY KEY); CREATE TABLE test4( a4 INT NOT NULL AUTO_INCREMENT PRIMARY KEY, b4 INT DEFAULT 0 ); delimiter | CREATE TRIGGER testref BEFORE INSERT ON test1 FOR EACH ROW BEGIN INSERT INTO test2 SET a2 = NEW.a1; DELETE FROM test3 WHERE a3 = NEW.a1; UPDATE test4 SET b4 = b4 + 1 WHERE a4 = NEW.a1; END; | delimiter ; INSERT INTO test3 (a3) VALUES (NULL), (NULL), (NULL), (NULL), (NULL), (NULL), (NULL), (NULL), (NULL), (NULL); INSERT INTO test4 (a4) VALUES (0), (0), (0), (0), (0), (0), (0), (0), (0), (0);
Suppose that you insert the following values into table
test1 as shown here:
mysql> INSERT INTO test1 VALUES (1), (3), (1), (7), (1), (8), (4), (4); Query OK, 8 rows affected (0.01 sec) Records: 8 Duplicates: 0 Warnings: 0
As a result, the four tables contain the following data:
mysql> SELECT * FROM test1; +------+ | a1 | +------+ | 1 | | 3 | | 1 | | 7 | | 1 | | 8 | | 4 | | 4 | +------+ 8 rows in set (0.00 sec) mysql> SELECT * FROM test2; +------+ | a2 | +------+ | 1 | | 3 | | 1 | | 7 | | 1 | | 8 | | 4 | | 4 | +------+ 8 rows in set (0.00 sec) mysql> SELECT * FROM test3; +----+ | a3 | +----+ | 2 | | 5 | | 6 | | 9 | | 10 | +----+ 5 rows in set (0.00 sec) mysql> SELECT * FROM test4; +----+------+ | a4 | b4 | +----+------+ | 1 | 3 | | 2 | 0 | | 3 | 1 | | 4 | 2 | | 5 | 0 | | 6 | 0 | | 7 | 1 | | 8 | 1 | | 9 | 0 | | 10 | 0 | +----+------+ 10 rows in set (0.00 sec)