mysql怎么删除重复数据并且保留其中一条

现在有张加班表,一个人同一天只允许申请一次加班,就是加班日期和userid相同的数据应该只有一条,但是现在由于之前没有做限制,导致很多数据重复

怎么通过sql删掉重复数据

思路:先查询重复数据的最大id(唯一一条),然后查询出所有重复数据,并且id不等于最大id的数据,然后删除即可

分解成几步,这样思路就相对清晰了

1.首先要知道怎么查询重复数据 就是SPONSOR_ID和datetime相同的数据

SELECT
		id 
	FROM
		t_yszg_grsq q 
	WHERE
		q.FORM_CODE = 303 
		AND q.FINISH_STATUS = 3 
		AND q.DELFLAG = 1 
	GROUP BY
		q.SPONSOR_ID,
		q.datetime 
	HAVING
		count(*) > 1

先查询重复数据的id

在查询最大的id就简单了,稍微改下

SELECT
			max( ID ) AS maxid 
		FROM
			t_yszg_grsq q 
		WHERE
			q.FORM_CODE = 303 
			AND q.FINISH_STATUS = 3 
			AND q.DELFLAG = 1 
		GROUP BY
			q.SPONSOR_ID,
			q.datetime 
		HAVING
			count(*) > 1

2.查询出重复的数据的SPONSOR_ID和datetime,

SELECT
	t.SPONSOR_ID,
	t.datetime 
FROM
	(
	SELECT
		q.SPONSOR_ID,
		q.datetime 
	FROM
		t_yszg_grsq q 
	WHERE
		q.FORM_CODE = 303 
		AND q.FINISH_STATUS = 3 
		AND q.DELFLAG = 1 
	GROUP BY
		q.SPONSOR_ID,
		q.datetime 
	HAVING
	count( 1 ) > 1 
	) AS t;

3.然后查询出SPONSOR_ID和datetime等于上面的数据,那么就可以查询出所有重复的数据,然后再删除不是最大id的数据

SELECT
	id 
FROM
	t_yszg_grsq 
WHERE
	( SPONSOR_ID, datetime ) IN (
	SELECT AT
		.SPONSOR_ID,
		AT.datetime 
	FROM
		(
		SELECT
			SPONSOR_ID,
			datetime 
		FROM
			t_yszg_grsq 
		WHERE
			FORM_CODE = 303 
			AND FINISH_STATUS = 3 
			AND DELFLAG = 1 
		GROUP BY
			SPONSOR_ID,
			datetime 
		HAVING
			count(*) > 1 
		) AT 
	)

4.现在就简单了,只要删除在3中不在1中的数据即可,逻辑删除DELFLAG=0

SELECT
	id 
FROM
	t_yszg_grsq 
WHERE
	( SPONSOR_ID, datetime ) IN (
	SELECT AT
		.SPONSOR_ID,
		AT.datetime 
	FROM
		(
		SELECT
			SPONSOR_ID,
			datetime 
		FROM
			t_yszg_grsq 
		WHERE
			FORM_CODE = 303 
			AND FINISH_STATUS = 3 
			AND DELFLAG = 1 
		GROUP BY
			SPONSOR_ID,
			datetime 
		HAVING
			count(*) > 1 
		) AT 
	) 
	AND ID NOT IN (
	SELECT
		mt.maxid 
	FROM
		(
		SELECT
			max( ID ) AS maxid 
		FROM
			t_yszg_grsq q 
		WHERE
			q.FORM_CODE = 303 
			AND q.FINISH_STATUS = 3 
			AND q.DELFLAG = 1 
		GROUP BY
			q.SPONSOR_ID,
			q.datetime 
		HAVING
			count(*) > 1 
		) AS mt 
	);

sql看起来复杂,分解成几步,思路清晰,其实也很简单