[SQL]SQL Server或MySQL等数据库中如何使用SQL脚本查找出多列重复的数据呢？

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在SQL Server或MySQL等这样的关系性数据库中，用一个字段很容易找到重复的数据记录，比如有以下的示例数据：

ID   NAME   EMAIL
1    John   asd@asd.com
2    Sam    asd@asd.com
3    Tom    asd@asd.com
4    Bob    bob@asd.com
5    Tom    asd@asd.com

查询重复记录的SQL脚本语句如下：

SELECT email, COUNT(email)
FROM users
GROUP BY email
HAVING COUNT(email) > 1

以上查询会返回John, Sam, Tom，因为是以email字段进行分组的，但现在需要找出name和email字段都相同的重复记录，应该如何编写SQL语句呢？

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Rector

2021-11-07 提问

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2 个回答

将name字段也添加到分组GROUP BY中，如下：

SELECT
    name, email, COUNT(*)
FROM
    users
GROUP BY
    name, email
HAVING COUNT(*) > 1

Rector

2021-11-07 回答

这里提供一个包含测试数据脚本的示例(SQL Server 2014)：

declare @YourTable table (id int, name varchar(10), email varchar(50))

INSERT @YourTable VALUES (1,'John','John-email')
INSERT @YourTable VALUES (2,'John','John-email')
INSERT @YourTable VALUES (3,'fred','John-email')
INSERT @YourTable VALUES (4,'fred','fred-email')
INSERT @YourTable VALUES (5,'sam','sam-email')
INSERT @YourTable VALUES (6,'sam','sam-email')

SELECT
    name,email, COUNT(*) AS CountOf
    FROM @YourTable
    GROUP BY name,email
    HAVING COUNT(*)>1

输入结果：

name       email       CountOf
---------- ----------- -----------
John       John-email  2
sam        sam-email   2

(2 行受影响)

如果想查询并返回哪些是重复的数据项，则：

SELECT
    y.id,y.name,y.email
    FROM @YourTable y
        INNER JOIN (SELECT
                        name,email, COUNT(*) AS CountOf
                        FROM @YourTable
                        GROUP BY name,email
                        HAVING COUNT(*)>1
                    ) dt ON y.name=dt.name AND y.email=dt.email

返回结果：

id          name       email
----------- ---------- ------------
1           John       John-email
2           John       John-email
5           sam        sam-email
6           sam        sam-email

(4 行受影响)

如果要删除重复的数据项，则：

DELETE d
    FROM @YourTable d
        INNER JOIN (SELECT
                        y.id,y.name,y.email,ROW_NUMBER() OVER(PARTITION BY y.name,y.email ORDER BY y.name,y.email,y.id) AS RowRank
                        FROM @YourTable y
                            INNER JOIN (SELECT
                                            name,email, COUNT(*) AS CountOf
                                            FROM @YourTable
                                            GROUP BY name,email
                                            HAVING COUNT(*)>1
                                        ) dt ON y.name=dt.name AND y.email=dt.email
                   ) dt2 ON d.id=dt2.id
        WHERE dt2.RowRank!=1
SELECT * FROM @YourTable

输出结果：

id          name       email
----------- ---------- --------------
1           John       John-email
3           fred       John-email
4           fred       fred-email
5           sam        sam-email

(4 行受影响)

Rector

2021-11-07 回答

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