MySQL触发器

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进行数据库移植，SQL Server=>MySQL。SQL Server上有如下的Trigger

SET QUOTED_IDENTIFIER ON 
GO
SET ANSI_NULLS ON 
GO
ALTER  TRIGGER [trg_risks] ON dbo.projectrisk
FOR INSERT, UPDATE
AS
BEGIN
UPDATE projectrisk
   SET classification =
   case    
	when  calc>= 9 then 3
	when calc <9 and calc>=4 then 2
  	when calc <4 then 1
   end 
   from (select inserted.id, inserted.possibility*inserted.severity  as calc from inserted) as T1
   where projectrisk.id = T1.id
END
GO
SET QUOTED_IDENTIFIER OFF 
GO
SET ANSI_NULLS ON 
GO

简单了解了下MySQL中，Trigger的语法。

# 创建
CREATE TRIGGER <触发器名称>
{ BEFORE | AFTER }
{ INSERT | UPDATE | DELETE }
ON <表名称>
FOR EACH ROW
<触发器SQL语句>

# 删除
DROP TRIGGER <触发器名称>

注：创建触发器需要CREATE TRIGGER权限。（HeidiSQL中执行Trigger语句会有bug）

由于MySQL中的每个触发器只能针对一个动作，所以本次移植就需要创建两个触发器。对于发生变更的行，在触发器中可以用 NEW 来代替。
下边的触发器有什么问题吗？

delimiter &&
CREATE TRIGGER trg_risks_insert
AFTER INSERT ON `projectrisk`
FOR EACH ROW
UPDATE projectrisk SET classification = CASE
WHEN possibility*severity>=9 THEN 3
WHEN possibility*severity <9 AND possibility*severity>=4 THEN 2
WHEN possibility*severity <4 THEN 1
END
WHERE id = new.id;
&&
CREATE TRIGGER trg_risks_update
AFTER UPDATE ON `projectrisk`
FOR EACH ROW
UPDATE projectrisk SET classification = CASE
WHEN possibility*severity>=9 THEN 3
WHEN possibility*severity <9 AND possibility*severity>=4 THEN 2
WHEN possibility*severity <4 THEN 1
END
WHERE id = new.id;
&&
delimiter ;

问题就是，没有考虑到触发器中的修改也会触发触发器，进入了死循环。做了如下修改后，终于OK了。

delimiter &&
CREATE TRIGGER trg_risks_insert
BEFORE INSERT ON `projectrisk`
FOR EACH ROW
BEGIN
  SET new.classification = CASE
  WHEN new.possibility*new.severity>=9 THEN 3
  WHEN new.possibility*new.severity <9 AND new.possibility*new.severity>=4 THEN 2
  WHEN new.possibility*new.severity <4 THEN 1
  END;
END
&&
CREATE TRIGGER trg_risks_update
BEFORE UPDATE ON `projectrisk`
FOR EACH ROW
BEGIN
  SET new.classification = CASE
  WHEN new.possibility*new.severity>=9 THEN 3
  WHEN new.possibility*new.severity <9 AND new.possibility*new.severity>=4 THEN 2
  WHEN new.possibility*new.severity <4 THEN 1
  END;
END
&&
delimiter ;