Question

示例数据库：

ID StudentName StudentClass
1  John        A
2  John        B
3  Peter       A
4  John        A
5  John        B

我希望结果应该是

ID StudentName StudentClass
1  John        A
2  John        B
3  Peter       A

声明

DELETE FROM Student
 WHERE ID NOT IN (SELECT * 
                    FROM (SELECT MIN(n.ID)
                            FROM Student n
                        GROUP BY n.StudentName) x)

如何在课程 A＆amp;上保留 John 名称？乙

Answer 1

DELETE a FROM Student a
LEFT JOIN
    (
        SELECT MIN(ID) AS minid
        FROM Student
        GROUP BY StudentName, StudentClass
    ) b ON a.id = b.minid
WHERE
    b.minid IS NULL

Answer 2

禁止甚至插入此类重复项的更好方法是多列唯一索引（它也会优化您的搜索）。方法如下：

ALTER TABLE `Student`
  ADD UNIQUE INDEX `idx` (`StudentName`, `StudentClass`)

Answer 3

你应该能够加入Students对抗自身，使用JOIN谓词来确保JOIN匹配重复的学生，并删除连接的行：

DELETE 
  duplicate_students.* 
FROM Students JOIN Students as duplicate_students 
  ON Students.StudentName = duplicate_students.StudentName
  AND Students.StudentClass = duplicate_students.StudentClass
  AND duplicate_students.ID > Students.ID

注意：请先备份您的数据;我对丢失的数据不负任何责任:-)这是一个概念性的想法，尚未经过测试。

Answer 4

这应该有效：

DELETE S FROM Student S
INNER JOIN(
    SELECT MIN(ID) AS ID,StudentName,StudentClass FROM Student
    GROUP BY StudentName,StudentClass
) S2 ON S.ID != S2.ID AND S.StudentName = S2.StudentName AND S.StudentClass = S2.StudentClass

它基本上从子查询中的所有重复记录中选择最小ID。然后我们只删除与该类和名称匹配的所有内容，但是我们与最小标识不匹配，因此在一天结束时，我们保留（大概）第一条记录不重复并根除休息。

如何使用group by删除重复值

4 个答案: