如何通过

Question

示例数据库中删除与组重复值：

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

我想结果应该是

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

Statment

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

如何保持约翰姓名A & B？

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

你应该能够加入Students对自身，与JOIN谓词确保连接匹配重复的学生，并删除join'd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：请您先备份您的数据;我对丢失的数据不承担任何责任:-)这是一个概念性的想法，尚未经过测试。

Answer 3

一个更好的方法来禁止即使是这样重复的插入会多列唯一索引（这将优化您的搜索太）。这里是如何：

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

Answer 4

这应该工作：

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 

它基本上选择最小ID出来的子查询中的所有重复的记录。然后我们简单地删除与Class和Name匹配的所有内容，但是我们不匹配最小ID，因此在一天结束时，我们保留（假定）第一条记录不符合重复记录并根除其余记录。