查询user表中，user_name字段值重复的数据及重复次数

select user_name,count(*) as count from user group by user_name having count>1

sql查询某字段的相同值:

SELECT * FROM table WHERE col in (SELECT col FROM table GROUP BY col HAVING COUNT (col) >1)

顺带说一下where和having：

select * from table
where ···(只能对分组前的属性进行筛选)
group by ···(按某个字段分组)
having ···(只能对分组后的每个组的整体属性进行筛选，用聚合函数体现)
--不使用group by就默认表的整体为一组