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一、概述

      在ANSI SQL中，如果使用了group by子句，出现在select子句中的必须是统计函数或group by子句中的列名称，可以结合having子句过滤掉一些不符合条件的结果。

二、单字段

Select 重复字段 From 表 Group By 重复字段 Having Count(*)>1

count(*)与count(1) 其实没有什么差别，用哪个都可以

count(*)与count（列名）的区别：

count(*)将返回表格中所有存在的行的总数包括值为null的行，然而count(列名)将返回表格中除去null以外的所有行的总数(有默认值的列也会被计入）

1、查询全部重复数据

Select * From 表 Where 重复字段 In (Select 重复字段 From 表 Group By 重复字段 Having Count(*)>1)

2、删除多余重复，只保留一条

DELETE
FROM
	dept
WHERE
	dname IN (
		SELECT
			t.dname
		FROM
			(
				SELECT
					dname
				FROM
					dept
				GROUP BY
					dname
				HAVING
					count(1) > 1
			) t
	)
AND deptno NOT IN (
SELECT
	dt.mindeptno
FROM
	(
		SELECT
			min(deptno) AS mindeptno
		FROM
			dept
		GROUP BY
			dname
		HAVING
			count(1) > 1
	) dt
)

--方式二
DELETE
FROM
	dept
WHERE
	deptno NOT IN (
		SELECT
			dt.minno
		FROM
			(
				SELECT
					MIN(deptno) AS minno
				FROM
					dept
				GROUP BY
					dname
			) dt
	)

--方式三
DELETE 
FROM
	table_name AS ta 
WHERE
	ta.唯一键 <> (
SELECT
	t.maxid 
FROM
	( SELECT max( tb.唯一键 ) AS maxid FROM table_name AS tb WHERE ta.判断重复的列 = tb.判断重复的列 ) t 
	);

三、多字段

单个字段的如果会了，多个字段也非常简单。就是将group by 的字段增加为你想要的即可。

此处只写一个，其他方法请仿照一个字段的写即可。

DELETE
FROM
	dept
WHERE
	(dname, db_source) IN (
		SELECT
			t.dname,
			t.db_source
		FROM
			(
				SELECT
					dname,
					db_source
				FROM
					dept
				GROUP BY
					dname,
					db_source
				HAVING
					count(1) > 1
			) t
	)
AND deptno NOT IN (
	SELECT
		dt.mindeptno
	FROM
		(
			SELECT
				min(deptno) AS mindeptno
			FROM
				dept
			GROUP BY
				dname,
				db_source
			HAVING
				count(1) > 1
		) dt
)