本文案例以MySQL5.7作為數(shù)據(jù)庫環(huán)境。

重復數(shù)據(jù)產(chǎn)生的原因有多種，比如系統(tǒng)存在bug、重復提交、需求調(diào)整（原來允許重復的內(nèi)容現(xiàn)在不允許重復了）... 原因就不一一列覺了，這里用實例來分析怎么解決重復數(shù)據(jù)的問題。

在另一篇《MySQL實戰(zhàn)》的用戶表中準備以下數(shù)據(jù)

mysql> select id,username,mobile from t_user;
+-------+----------+-------------+
| id    | username | mobile      |
+-------+----------+-------------+
| 10001 | user1    | 13900000001 |
| 10002 | user2    | NULL        |
| 10003 | user3    | NULL        |
| 10004 | user4    | NULL        |
| 10005 | user5    | NULL        |
| 10006 | user6    | 13900000001 |
+-------+----------+-------------+

現(xiàn)在需要檢查用戶表中手機號mobile重復的數(shù)據(jù)，可以利用聚合函數(shù)count()按mobile字段group by找到需要的結(jié)果。

# 查詢找到出現(xiàn)重復的手機號
mysql> select mobile,count(1) as c from t_user where mobile is not null group by mobile having c > 1;
+-------------+---+
| mobile      | c |
+-------------+---+
| 13900000001 | 2 |
+-------------+---+

接下來根據(jù)需要對重復的手機號進行處理，比如將id較大的記錄中的手機號設(shè)為null。

我們按照要求一步一步來完善上面的sql，既然要對id較大的記錄處理，那么久需要找到id最小的記錄

# mim(id)
mysql> select mobile,count(1) as c,min(id) as min_id from t_user where mobile is not null group by mobile having c > 1;
+-------------+---+--------+
| mobile      | c | min_id |
+-------------+---+--------+
| 13900000001 | 2 |  10001 |
+-------------+---+--------+

找到最小id后，將t_user與查詢結(jié)果join，執(zhí)行update動作。

# update ... where id > ...
mysql> update t_user as u
    -> join (
    -> select mobile,count(1) as c,min(id) as min_id from t_user where mobile is not null group by mobile having c > 1
    -> ) as a on u.mobile = a.mobile
    -> set mobile = null
    -> where u.id > a.min_id;
Query OK, 1 row affected (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 0

提示執(zhí)行成功，最后檢查下是否達到預期效果。

# 查詢是否存在mobile重復的記錄
mysql> select mobile,count(1) as c from t_user where mobile is not null group by mobile having c > 1;
Empty set (0.00 sec)
# 再通過直觀方式再次驗證
mysql> select id,username,mobile from t_user;
+-------+----------+-------------+
| id    | username | mobile      |
+-------+----------+-------------+
| 10001 | user1    | 13900000001 |
| 10002 | user2    | NULL        |
| 10003 | user3    | NULL        |
| 10004 | user4    | NULL        |
| 10005 | user5    | NULL        |
| 10006 | user6    | NULL        |
+-------+----------+-------------+
6 rows in set (0.00 sec)