MYSQL入门学习之十一：触发器的基本操作


触发器是MySQL响应以下任意语句而自动执行的一条MySQL语句（或位于BEGIN和END语句之间的一组语句）：

DELETE；

INSERT；

UPDATE；

使用触发器，需要MySQL5或之后的版本支持。

一、触发器基本操作

1、创建触发器

创建触发器时，需要给出4条信息：

唯一的触发器名；(虽然MySQL5允许不同的表上的触发器名称相同，但一般最好不要这么做。)

触发器关联的表；

触发器响应的事件；

触发器何时执行；

语法结构：

create trigger trigger_name (BEFORE|AFTER) (delete|update|insert) on table_name

for each row

BEGIN

要触发的sql语句;

END;

示例：

[sql]
view plaincopyprint?

mysql> delimiter |
mysql> create trigger t_trig before insert on t_goods for each row
-> begin
-> set NEW.add_date = current_date();
-> end;
-> |
mysql> delimiter ;
mysql> insert into t_goods(id,goods_name,quantity)
-> values(1,'apple',50);
-> |
mysql> select * from t_goods;
-> |
+------+------------+----------+------------+
| id | goods_name | quantity | add_date |
+------+------------+----------+------------+
| 1 | apple | 50 | 2012-12-12 |
+------+------------+----------+------------+

mysql> delimiter |
mysql> create trigger t_trig before insert on t_goods for each row
-> begin
->     set NEW.add_date = current_date();
-> end;
-> |
mysql> delimiter ;
mysql> insert into t_goods(id,goods_name,quantity)
-> values(1,'apple',50);
-> |
mysql> select * from t_goods;
-> |
+------+------------+----------+------------+
| id   | goods_name | quantity | add_date   |
+------+------------+----------+------------+
|    1 | apple      |       50 | 2012-12-12 |
+------+------------+----------+------------+

2、删除触发器

DROP TRIGGER [schema_name.]trigger_name；

示例：

mysql> DROP TRIGGER t_trig;

3、查看触发器

SHOW TRIGGERS [ FROM DBNAME [ like '' ] ];

示例：

[sql]
view plaincopyprint?

mysql> show triggers;
+---------+--------+---------+------------------------------------------------------------------------------------------
| Trigger | Event | Table | Statement
+---------+--------+---------+------------------------------------------------------------------------------------------
| t_trig | INSERT | t_goods | begin
set NEW.add_date = current_date();
end | BEFORE | NULL | | root@localhost | latin1 | latin1_swedish_ci | latin1_swedish_ci |
+---------+--------+---------+------------------------------------------------------------------------------------------
mysql> show triggers from test;
+---------+--------+---------+------------------------------------------------------------------------------------------
| Trigger | Event | Table | Statement
+---------+--------+---------+------------------------------------------------------------------------------------------
| t_trig | INSERT | t_goods | begin
set NEW.add_date = current_date();
end | BEFORE | NULL | | root@localhost | latin1 | latin1_swedish_ci | latin1_swedish_ci |
+---------+--------+---------+------------------------------------------------------------------------------------------
mysql> show triggers from test like 't%';
+---------+--------+---------+------------------------------------------------------------------------------------------
| Trigger | Event | Table | Statement
+---------+--------+---------+------------------------------------------------------------------------------------------
| t_trig | INSERT | t_goods | begin
set NEW.add_date = current_date();
end | BEFORE | NULL | | root@localhost | latin1 | latin1_swedish_ci | latin1_swedish_ci |
+---------+--------+---------+------------------------------------------------------------------------------------------

mysql> show triggers;
+---------+--------+---------+------------------------------------------------------------------------------------------
| Trigger | Event  | Table   | Statement
+---------+--------+---------+------------------------------------------------------------------------------------------
| t_trig  | INSERT | t_goods | begin
set NEW.add_date = current_date();
end | BEFORE | NULL    |          | root@localhost | latin1               | latin1_swedish_ci    | latin1_swedish_ci  |
+---------+--------+---------+------------------------------------------------------------------------------------------
mysql> show triggers from test;
+---------+--------+---------+------------------------------------------------------------------------------------------
| Trigger | Event  | Table   | Statement
+---------+--------+---------+------------------------------------------------------------------------------------------
| t_trig  | INSERT | t_goods | begin
set NEW.add_date = current_date();
end | BEFORE | NULL    |          | root@localhost | latin1               | latin1_swedish_ci    | latin1_swedish_ci  |
+---------+--------+---------+------------------------------------------------------------------------------------------
mysql> show triggers from test like 't%';
+---------+--------+---------+------------------------------------------------------------------------------------------
| Trigger | Event  | Table   | Statement
+---------+--------+---------+------------------------------------------------------------------------------------------
| t_trig  | INSERT | t_goods | begin
set NEW.add_date = current_date();
end | BEFORE | NULL    |          | root@localhost | latin1               | latin1_swedish_ci    | latin1_swedish_ci  |
+---------+--------+---------+------------------------------------------------------------------------------------------

二、注意事项

1、只有表支持触发器，视图及临时表都不支持；

2、每个表最多支持6个触发器；

3、单一触发器不能与多个事件或多个表关联；

4、对于INSERT而言，新增的行用NEW来表示，行中的每一列的值，用NEW.列名来表示；

对于DELETE而言，删除的行用OLD来表示；

对于UPDATE而言，更新前的行用OLD来表示，更新后的行用NEW来表示；