MySQL CREATE TRIGGER (1)
CREATE TRIGGER语法
CREATE TRIGGER trigger_name trigger_time trigger_event
ON tbl_name FOR EACH ROW trigger_stmt
触发程序是与表有关的命名数据库对象，当表上出现特定事件时，将激活该对象。
触发程序与命名为tbl_name的表相关。tbl_name必须引用永久性表。不能将触发程序与TEMPORARY表或视图关联起来。
trigger_time是触发程序的动作时间。它可以是BEFORE或AFTER，以指明触发程序是在激活它的语句之前或之后触发。
trigger_event指明了激活触发程序的语句的类型。trigger_event可以是下述值之一：
· INSERT：将新行插入表时激活触发程序，例如，通过INSERT、LOAD DATA和REPLACE语句。
· UPDATE：更改某一行时激活触发程序，例如，通过UPDATE语句。
· DELETE：从表中删除某一行时激活触发程序，例如，通过DELETE和REPLACE语句。
请注意，trigger_event与以表操作方式激活触发程序的SQL语句并不很类似，这点很重要。例如，关于INSERT的BEFORE触发程序不仅能被INSERT语句激活，也能被LOAD DATA语句激活。
可能会造成混淆的例子之一是INSERT INTO .. ON DUPLICATE UPDATE ...语法：BEFORE INSERT触发程序对于每一行将激活，后跟AFTER INSERT触发程序，或BEFORE UPDATE和AFTER UPDATE触发程序，具体情况取决于行上是否有重复键。
对于具有相同触发程序动作时间和事件的给定表，不能有两个触发程序。例如，对于某一表，不能有两个BEFORE UPDATE触发程序。但可以有1个BEFORE UPDATE触发程序和1个BEFORE INSERT触发程序，或1个BEFORE UPDATE触发程序和1个AFTER UPDATE触发程序。
trigger_stmt是当触发程序激活时执行的语句。如果你打算执行多个语句，可使用BEGIN ... END复合语句结构。这样，就能使用存储子程序中允许的相同语句。
在MySQL 5.6.22 为版本的数据库举例说明如下:
mysql> CREATE TABLE test1(a1 INT);
Query OK, 0 rows affected (0.26 sec)
mysql> CREATE TABLE test2(a2 INT);
Query OK, 0 rows affected (0.24 sec)
mysql> CREATE TABLE test3(a3 INT NOT NULL AUTO_INCREMENT PRIMARY KEY);
Query OK, 0 rows affected (0.34 sec)
mysql> CREATE TABLE test4(
-> a4 INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
-> b4 INT DEFAULT 0
-> );
Query OK, 0 rows affected (0.32 sec)
mysql> DELIMITER |
mysql> 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
-> |
Query OK, 0 rows affected (0.06 sec)
mysql> DELIMITER ;
mysql> INSERT INTO test3 (a3) VALUES
-> (NULL), (NULL), (NULL), (NULL), (NULL),
-> (NULL), (NULL), (NULL), (NULL), (NULL);
Query OK, 10 rows affected (0.01 sec)
Records: 10 Duplicates: 0 Warnings: 0
mysql> INSERT INTO test4 (a4) VALUES
-> (0), (0), (0), (0), (0), (0), (0), (0), (0), (0);
Query OK, 10 rows affected (0.00 sec)
Records: 10 Duplicates: 0 Warnings: 0
mysql> select * from test1;
Empty set (0.00 sec)
mysql> select * from test2;
Empty set (0.00 sec)
mysql> select * from test3;
+----+
| a3 |
+----+
| 1 |
| 2 |
| 3 |
| 4 |
| 5 |
| 6 |
| 7 |
| 8 |
| 9 |
| 10 |
+----+
10 rows in set (0.00 sec)
mysql> select * from test4;
+----+------+
| a4 | b4 |
+----+------+
| 1 | 0 |
| 2 | 0 |
| 3 | 0 |
| 4 | 0 |
| 5 | 0 |
| 6 | 0 |
| 7 | 0 |
| 8 | 0 |
| 9 | 0 |
| 10 | 0 |
+----+------+
10 rows in set (0.00 sec)
mysql> INSERT INTO test1 VALUES
-> -> (1), (3), (1), (7), (1), (8), (4), (4);
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that
corresponds to your MySQL server version for the right syntax to use near '-> (1
), (3), (1), (7), (1), (8), (4), (4)' at line 2
mysql> INSERT INTO test1 VALUES (1), (3), (1), (7), (1), (8), (4), (4);
Query OK, 8 rows affected, 1 warning (0.22 sec)
Records: 8 Duplicates: 0 Warnings: 1
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)
mysql> select * from test3;
+----+
| a3 |
+----+
| 2 |
| 5 |
| 6 |
| 9 |
| 10 |
+----+
5 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 test1;
+------+
| a1 |
+------+
| 1 |
| 3 |
| 1 |
| 7 |
| 1 |
| 8 |
| 4 |
| 4 |
+------+
8 rows in set (0.00 sec)
mysql>
