mysql 触发器 for each row 理解_“for each row”如何在mysql中的触发器中工作？

FOR EACH ROW表示每个匹配行的更新或删除.

除非查询中有where条件,否则触发器主体不会遍历整个表数据.

下面演示了一个工作示例：

创建样本表：

drop table if exists tbl_so_q23374151;

create table tbl_so_q23374151 ( i int, v varchar(10) );

-- set test data

insert into tbl_so_q23374151

values (1,'one'),(2,'two' ),(3,'three'),(10,'ten'),(11,'eleven');

-- see current data in table**:

select * from tbl_so_q23374151;

+------+--------+

| i | v |

+------+--------+

| 1 | one |

| 2 | two |

| 3 | three |

| 10 | ten |

| 11 | eleven |

+------+--------+

5 rows in set (0.00 sec)

用于记录触发器主体中循环计数的样本表：

-- let us record, loop count of trigger, in a table

drop table if exists tbl_so_q23374151_rows_affected;

create table tbl_so_q23374151_rows_affected( i int );

select count(*) as rows_affected from tbl_so_q23374151_rows_affected;

+---------------+

| rows_affected |

+---------------+

| 0 |

+---------------+

定义删除触发器：

drop trigger if exists trig_bef_del_on_tbl_so_q23374151;

delimiter //

create trigger trig_bef_del_on_tbl_so_q23374151 before delete on tbl_so_q23374151

for each row begin

set @cnt = if(@cnt is null, 1, (@cnt+1));

/* for cross checking save loop count */

insert into tbl_so_q23374151_rows_affected values ( @cnt );

end;

//

delimiter ;

现在,测试删除操作：

delete from tbl_so_q23374151 where i like '%1%';

-- now let us see what the loop count was

select @cnt as 'cnt';

+------+

| cnt |

+------+

| 3 |

+------+

现在,检查主表上的触发效果：

-- now let us see the table data

select * from tbl_so_q23374151;

+------+-------+

| i | v |

+------+-------+

| 2 | two |

| 3 | three |

+------+-------+

2 rows in set (0.00 sec)

select count(*) as rows_affected from tbl_so_q23374151_rows_affected;

+---------------+

| rows_affected |

+---------------+

| 3 |

+---------------+

1 row in set (0.00 sec)

总结

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