MySQL 資料庫查詢數據，過濾重覆數據保留一條數據---（MySQL中的row_number變相實現方法）

轉自： http://www.maomao365.com/?p=10564  摘要:

下文講述MySQL資料庫查詢重覆數據時，只保留一條數據的方法 
實現思路:
在MySQL資料庫中沒有row_number函數，所以我們需變向實現此功能。
 

--1.基礎數據表查看
mysql> select * from `maomao365.com`;
+-------+------------+
| keyId | infoB |
+-------+------------+
| 1 | mysql test |
| 129 | sql |
| 12913 | sql |
| 12913 | sql |
| 12913 | sql |
+-------+------------+
5 rows in set (0.00 sec)

---2 群組編號
mysql> select if(@keyId=a.keyId and @infoB=a.infoB,@r:=@r+1,@r:=1) as rowNumber,a.*,@keyId:=a.keyId,@infoB:=a.infoB
-> from (select @keyId:=null,@infoB:=null,@r:=0) r, (select keyId,infoB from `maomao365.com` order by keyId) a;
+-----------+-------+------------+-----------------+-----------------+
| rowNumber | keyId | infoB | @keyId:=a.keyId | @infoB:=a.infoB |
+-----------+-------+------------+-----------------+-----------------+
| 1 | 1 | mysql test | 1 | mysql test |
| 1 | 129 | sql | 129 | sql |
| 1 | 12913 | sql | 12913 | sql |
| 2 | 12913 | sql | 12913 | sql |
| 3 | 12913 | sql | 12913 | sql |
+-----------+-------+------------+-----------------+-----------------+
5 rows in set (0.00 sec)

---3 重覆數據只顯示一條
mysql> select * from (select if(@keyId=a.keyId and @infoB=a.infoB,@r:=@r+1,@r:=1) as rowNumber,a.*,@keyId:=a.keyId,@infoB:=a.infoB
-> from (select @keyId:=null,@infoB:=null,@r:=0) r, (select keyId,infoB from `maomao365.com` order by keyId) a
-> ) as t where t.rowNumber =1;
+-----------+-------+------------+-----------------+-----------------+
| rowNumber | keyId | infoB | @keyId:=a.keyId | @infoB:=a.infoB |
+-----------+-------+------------+-----------------+-----------------+
| 1 | 1 | mysql test | 1 | mysql test |
| 1 | 129 | sql | 129 | sql |
| 1 | 12913 | sql | 12913 | sql |
+-----------+-------+------------+-----------------+-----------------+
3 rows in set (0.00 sec)

定義臨時變數 @keyId,@infoB
每次查詢時 都重新對 @keyId,@infoB 賦值
在賦值前，都採用If對其進行判斷，如果當前行記錄信息和臨時變數中的值相同時，則作為相同數據@r變數加1，否則@r變數變為1
採用以上模式，可以將重覆的行記錄進行編號

最後對重新編號後的記錄，檢索出row_number等於1的信息