刪除重複在一列

Question

我有以下查詢基本上找到自己的用戶名列中的所有副本：

SELECT `username` 
FROM `instagram_user` 
GROUP BY `username` 
HAVING COUNT(*) >1 

如何刪除所有的重複，這樣只會讓我用一個唯一的用戶名中桌子？只要表中有一個唯一的用戶名，我不關心哪個實體被持久或刪除。

Answer 1

如果你不在乎什麼紀錄選擇然後只是添加一個唯一的約束，而使用IGNORE

ALTER IGNORE TABLE instagram_user ADD UNIQUE (username); 

這裏是SQLFiddle演示

和MySQL會爲你做這項工作。無論如何，您都希望擁有唯一的約束條件，以便將來您的表格不會出現重複。

或者你也可以做

DELETE t 
    FROM instagram_user t JOIN 
(
    SELECT username, MAX(id) id 
    FROM instagram_user 
    GROUP BY username 
    HAVING COUNT(*) > 1 
) q 
    ON t.username = q.username 
    AND t.id <> q.id 

這一次將只留下與最大ID一排具有重複的用戶名行。

這裏是SQLFiddle演示

Answer 2

不確定這是否適用於SQL Server，您可以嘗試在mysql中使用類似的代碼。

;With CteUsers AS(

SELECT *,ROW_NUMBER() OVER (PARTITION BY username Order by username) AS ROWID 
FROM(

SELECT PkId, `username` 
FROM `instagram_user` 
)tbltemp) 
SELECT * FROM CteUsers; 

這將導致如下

PkId username RowId 
1  xx  1 
2  xx  2 
.... 

然後刪除其中RowId的> 1

;隨着CteUsers AS（

SELECT *,ROW_NUMBER() OVER (PARTITION BY username Order by username) AS ROWID 
    FROM(

    SELECT PkId, `username` 
    FROM `instagram_user` 
    )tbltemp) 
    DELETE instagram_user WHERE PkId iN (SELECT PkId FROM CteUsers WHERE ROWID > 1); 

Answer 3

這會給你的副本（即你需要刪除的）...

select a.id, a.username from instagram_user a, instagram_user b 
where a.username = b.username and a.id <> b.id 
    and b.id = (select min(id) from instagram_user where username = a.username) 

所以DELETE會是這樣的......

delete from instagram_user where id in 
(select a.id from instagram_user a, instagram_user b 
    where a.username = b.username and a.id <> b.id 
     and b.id = (select min(c.id) from instagram_user c 
        where c.username = a.username))