如何“针对每一行"在MySQL触发器中工作? [英] How does "for each row" work in triggers in mysql?

问题描述

在mysql触发器中，当我在表A上执行更新后"，然后使用针对每一行"时，每次更新行时，它将为A中的每一行运行触发器的主体在A中，还是说将触发器应用于A中的每一行，然后如果一行被更新，它将仅运行该更新行的主体代码?

In mysql triggers, when I do a "after update" on table A and then use "for each row", will it run the body of the trigger for each row in A every time a row gets updated in A, or is it saying to apply the trigger to every row in A and then if a row gets updated, it will only run the body code for that updated row only?

谢谢

推荐答案

FOR EACH ROW 表示每个匹配行的 ，它可以 更新 或 删除 .

FOR EACH ROW means for each of the matched row that gets either updated or deleted.

除非查询中存在where条件，否则触发主体将不会遍历整个表数据.

Trigger body won't loop through the entire table data unless there is a where condition in the query.

一个工作示例如下所示:

A working example is demonstrated below:

创建示例表:

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)

用于在触发器主体中记录循环计数的样本表:

-- 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:

Define a delete trigger:

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:

Now, test a delete operation:

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)

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