查詢刪除重複項會產生錯誤

Question

我想根據客戶電話號碼從表中刪除重複項。下面正確內選擇查詢，選擇，我需要刪除所有重複的記錄，但是outter刪除查詢生成以下錯誤：

You can't specify target table 'customers' for update in FROM clause 

查詢：

DELETE FROM customers WHERE id IN (SELECT id from customers GROUP BY phone HAVING COUNT(phone) > 1) 

Answer 1

在update和delete語句，通常不能引用被修改的表。一個簡單的解決方案是使用一個join：

DELETE c 
    FROM customers c JOIN 
     (SELECT phone, COUNT(*) as cnt 
      FROM customers c 
      GROUP BY phone 
     ) p 
     ON c.phone = p.phone AND cnt > 1; 

請注意，此刪除表中的所有重複。通常情況下，你想保留其中之一：

DELETE c 
    FROM customers c LEFT JOIN 
     (SELECT phone, MIN(id) as minid 
      FROM customers c 
      GROUP BY phone 
     ) p 
     ON c.id = p.minid 
    WHERE p.phone IS NULL;