在實際錄入資料庫的過程中，如果資料量比較大的話，難免會因為一些原因，而錄入多條重複的記錄，那麼應該如何操作才能刪除重複行，並且保留一條id較大，或者較小的記錄呢。

        在本例中所用資料表結構如下所示tdb_goods

        表中資料重複如圖所示

        首先第一步，利用group by分組查出每組中數目大於2的（即重複記錄的）內容

mysql> SELECT goods_id,goods_name FROM tdb_goods GROUP BY goods_name HAVING COUN
T(goods_name)>=2;
+----------+------------------------------+
| goods_id | goods_name                   |
+----------+------------------------------+
|       20 | X3250 M4機架式伺服器 2583i14            |
|       19 | 商務雙肩揹包                          |
+----------+------------------------------+
2 rows in set (0.01 sec)
 

mysql> DELETE t1 FROM tdb_goods AS t1 LEFT JOIN( SELECT goods_id,goods_name FROM
 tdb_goods GROUP BY goods_name HAVING COUNT(goods_name)>=2) AS t2 ON t1.goods_na
me = t2.goods_name WHERE t1.goods_id>t2.goods_id;
Query OK, 2 rows affected (0.06 sec)
 

        如果想保留id較大的同理，如下所示

mysql> DELETE t1 FROM tdb_goods AS t1 LEFT JOIN( SELECT max(goods_id) AS goods_i
d,goods_name FROM tdb_goods GROUP BY goods_name HAVING COUNT(goods_name)>=2) AS
t2 ON t1.goods_name = t2.goods_name WHERE t1.goods_id<t2.goods_id;
Query OK, 2 rows affected (0.03 sec)
 

注關於表連線，如有疑問請檢視JOIN的用法。