MySQL中删除重复数据只保留一条

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本文介绍如何使用SQL语句从数据库中删除重复记录，并提供针对单字段及多字段的解决方案。同时，文章还提供了处理MySQL中特定错误的方法以及通过创建临时表进行数据处理的技巧。

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用SQL语句,删除掉重复项只保留一条

在几千条记录里,存在着些相同的记录,如何能用SQL语句,删除掉重复的呢
1、查找表中多余的重复记录，重复记录是根据单个字段（peopleId）来判断

SELECT
    *
FROM
    people
WHERE
    peopleId IN (
        SELECT
            peopleId
        FROM
            people
        GROUP BY
            peopleId
        HAVING
            count(peopleId) > 1
    )

2、删除表中多余的重复记录，重复记录是根据单个字段（peopleId）来判断，只留有rowid最小的记录

DELETE
FROM
    people
WHERE
    peopleName IN (
        SELECT
            peopleName
        FROM
            people
        GROUP BY
            peopleName
        HAVING
            count(peopleName) > 1
    )
AND peopleId NOT IN (
    SELECT
        min(peopleId)
    FROM
        people
    GROUP BY
        peopleName
    HAVING
        count(peopleName) > 1
)

上面的sql语句经测试会报错：MySQL 中 You can't specify target table '表名' for update in FROM clause错误。

（在MySQL中，写SQL语句的时候，可能会遇到You can't specify target table '表名' for update in FROM clause这样的错误，它的意思是说，不能先select出同一表中的某些值，再update这个表(在同一语句中)，即不能依据某字段值做判断再来更新某字段的值。）

解决办法：将下列查询出来的结果即需要删除的数据存入临时表里，在删除存在于临时表中的数据。

创建临时表存入下面sql查询出来的需要删除的数据。

SELECT
    peopleId
FROM
	people
WHERE
	peopleName IN (
	    SELECT
			peopleName
		FROM
			people
		GROUP BY
			peopleName
		HAVING
			count(peopleName) > 1
	)
	AND peopleId NOT IN (
		SELECT
			min(peopleId)
		FROM
			people
		GROUP BY
			peopleName
		HAVING
			count(peopleName) > 1
	)
)

然后再删除正式表中存在于临时表中的数据。

DELETE
FROM
	people
WHERE
	peopleId IN (
	    上面临时表中存在的数据
	)

3、查找表中多余的重复记录（多个字段）

SELECT
    *
FROM
    vitae a
WHERE
    (a.peopleId, a.seq) IN (
        SELECT
            peopleId,
            seq
        FROM
            vitae
        GROUP BY
            peopleId,
            seq
        HAVING
            count(*) > 1
    )

4、删除表中多余的重复记录（多个字段），只留有rowid最小的记录

DELETE
FROM
    vitae a
WHERE
    (a.peopleId, a.seq) IN (
        SELECT
            peopleId,
            seq
        FROM
            vitae
        GROUP BY
            peopleId,
            seq
        HAVING
            count(*) > 1
    )
AND rowid NOT IN (
    SELECT
        min(rowid)
    FROM
        vitae
    GROUP BY
        peopleId,
        seq
    HAVING
        count(*) > 1
)

5、查找表中多余的重复记录（多个字段），不包含rowid最小的记录

SELECT
    *
FROM
    vitae a
WHERE
    (a.peopleId, a.seq) IN (
        SELECT
            peopleId,
            seq
        FROM
            vitae
        GROUP BY
            peopleId,
            seq
        HAVING
            count(*) > 1
    )
AND rowid NOT IN (
    SELECT
        min(rowid)
    FROM
        vitae
    GROUP BY
        peopleId,
        seq
    HAVING
        count(*) > 1
)

6.消除一个字段的左边的第一位：

UPDATE tableName
SET [ Title ]= RIGHT ([ Title ],(len([ Title ]) - 1))
WHERE
    Title LIKE '村%'

7.消除一个字段的右边的第一位：

UPDATE tableName
SET [ Title ]= LEFT ([ Title ],(len([ Title ]) - 1))
WHERE
    Title LIKE '%村'

8.假删除表中多余的重复记录（多个字段），不包含rowid最小的记录

UPDATE vitae
SET ispass =- 1
WHERE
    peopleId IN (
        SELECT
            peopleId
        FROM
            vitae
        GROUP BY
            peopleId