mysql删除重复记录并且只保留一条

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

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）

删除表中多余重复试题并且只留1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

　　

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

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 
    );

来源    https://blog.csdn.net/n950814abc/article/details/82284838 

SQL 删除重复记录方法：https://www.cnblogs.com/shy1766IT/p/5185719.html