如何让相同的字段在SQL自连接中不出现两次？

Question

我想要做的故事自联接，使一对客户，但我不希望同样对出现两次，我用这个代码

SELECT DISTINCT A.customer_num AS ' num1' 
       , B.customer_num AS 'num2' 
       , A.customer_name AS 'name1' 
       , B.customer_name AS 'name2' 
       , A.city AS 'city' 
      FROM tbl_customer A 
       , tbl_customer B 
      WHERE A.customer_num <> B.customer_num 
       AND A.city = B.city ; 

它给我的记录，但对得到重复，这就是我得到的结果My Result，如1和第6对是相同的，我得到了 ，我正在使用MySQL Workbench。

Answer 1

由于您的自加入where条件是对称（即A.customer_num <> B.customer_num），每对将在输出中出现两次。无论记录是A还是B，因此将条件切换到不对称将解决该问题。

当你在它时，切换到使用ANSI连接;您也不再需要DISTINCT：

SELECT 
    A.customer_num AS 'num1' 
, B.customer_num AS 'num2' 
, A.customer_name AS 'name1' 
, B.customer_name AS 'name2' , 
A.city AS 'city' 
FROM tbl_customer A -- It does not matter if you use <or> below 
JOIN tbl_customer B ON A.customer_num > B.customer_num AND A.city = B.city