mysql——查詢重覆數據，及刪除重覆數據只保留一條數據

查詢 text 表中，user_name欄位值重覆的數據及重覆次數

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

刪除 text 表中，重覆出現的數據只保留 ID 最小的一條數據，沒有重覆的數據不刪除。

delete from text where DW_DATE=20171227 AND
 id not in( select id from (select max(id) as id,count(user_name) as count from text
WHERE DW_DATE=20171227
group by user_name having count =1 order by count desc) as tab)
AND
id not in( select id from (select max(id) as id,count(user_name) as count from text
WHERE DW_DATE=20171227
group by user_name having count >1 order by count desc) as tab)