查询删除重复数据，只保留一条记录：

1.根据单字段，查询表中该字段重复出现记录：

SELECT * FROM Table
WHERE Parameter IN
(SELECT Parameter FROM Table
GROUP BY Parameter HAVING COUNT(Parameter ) > 1)

分析：
SELECT Parameter FROM Table
GROUP BY Parameter HAVING COUNT(Parameter ) > 1
根据字段分组，查询出表中出现重复（字段数量>1）的字段集

-----------------------------华丽的分割线---------------------------------

2.根据单字段，删除表中该字段重复出现记录，保留最小主键ID

DELETE FROM Table
WHERE Parameter IN
(SELECT Parameter FROM Table
GROUP BY Parameter HAVING COUNT(Parameter) > 1)
AND ID NOT IN
(SELECT MIN(ID) FROM Table
GROUP BY Parameter HAVING COUNT(Parameter)>1)

-----------------------------华丽的分割线---------------------------------

3.根据多个字段查询表中重复的记录

SELECT * FROM Table a
WHERE EXISTS
(SELECT Para1 ,Para2 FROM Table b
WHERE a.Para1 = b.Para1 AND
a.Para1 = b.Para2 GROUP BY Para1 ,Para2
HAVING COUNT(*) > 1 )

-----------------------------华丽的分割线---------------------------------

4.根据多个字段 删除表中重复的记录，保留最小主键记录

DELETE A
FROM Table A ,
(SELECT Para1,Para2 FROM Table
GROUP BY Para1,Para2
HAVING COUNT(*) > 1 ) B
WHERE A.Para1=B.Para1 AND a.Para2= b.Para2
AND A.ID NOT IN (SELECT MIN(ID) AS ID
FROM Table
GROUP BY Para1,Para2
HAVING COUNT(*) > 1)

-----------------------------华丽的分割线---------------------------------

5.查询多个字段重复记录，不包含最小主键ID

SELECT * FROM Table a
WHERE EXISTS
(SELECT Para1,Para2 FROM Table b
WHERE a.Para1 = b.Para1 AND a. Para2 = b.Para2
GROUP BY Para1,Para2
HAVING COUNT(*) > 1)
AND ID NOT IN
(SELECT MIN(ID) FROM Table
GROUP BY Para1,Para2
HAVING COUNT(*)>1)

-----------------------------华丽的分割线---------------------------------
转：https://blog.csdn.net/xuejiaxingq/article/details/75104618