MySql數據庫去重

shoes表結構

 

 在此表中，shoes_name可能有重復，本篇博客記錄如何去除重復數據。

1.首先要知道哪些數據是重復的, 可用group by 聚集函數找到:

SELECT shoes_name,count(*) from shoes GROUP BY shoes_name having COUNT(*)>1

注：having 一般和group連用，用來限制查到的結果，這里的意思是將shoes表按shoes_name組，count(*)計算每組的條數，hiving限制顯示條數大於1的結果，即有重復的數據。

　　

2.根據第一步中獲得的shoes_name來獲得所有重復的數據

SELECT * from shoes WHERE shoes_name IN(
    SELECT * from (
        SELECT shoes_name from shoes GROUP BY shoes_name having COUNT(*)>1) t1
    )

 

 

3.因為刪除時我們要保留id最小的數據行，所以我們要查找最小的id。

SELECT id from shoes WHERE id in (
    SELECT * from (
        select MIN(id) from shoes GROUP BY shoes_name having COUNT(*)>1
    )t2
)

 

 

4.刪除這些重復數據，只保留最小的table_id

DELETE from shoes where shoes_name IN(
    SELECT * from(
        SELECT shoes_name FROM shoes GROUP BY shoes_name having COUNT(*)>1
    )t1
)
AND id not IN(
    SELECT * from (
        select MIN(id) from shoes GROUP BY shoes_name having COUNT(*)>1
    )t2
)