SQL UNION的兩個查詢，重複的列名錯誤

Question

我需要兩個查詢的UNION，他們每個人分別工作，但不在一起，我得到錯誤：重複列名zipcode_id，請大家幫忙。

(SELECT * FROM 
    ((SELECT * FROM jobs AS j LEFT JOIN zipcode AS z ON z.zipcode_id=j.zipcode_id WHERE 1 AND source='student' 
       ORDER BY postdate DESC LIMIT 20) ORDER BY search_order DESC) 
s1) 
    UNION ALL 
(SELECT * FROM 
     ( (SELECT * FROM jobs AS j LEFT JOIN zipcode AS z ON z.zipcode_id=j.zipcode_id WHERE 1 AND source='manager' 
       ORDER BY postdate DESC LIMIT 30, 1000000) ORDER BY postdate DESC) 
s2) 

Answer 1

您可能需要爲每個子查詢使用不同的別名。這應該工作：

(SELECT * FROM 
    ((SELECT j1.* FROM jobs AS j1 LEFT JOIN zipcode AS z1 ON z1.zipcode_id=j1.zipcode_id WHERE 1 AND source='student' 
       ORDER BY postdate DESC LIMIT 20) ORDER BY search_order DESC) s1) UNION ALL 
(SELECT * FROM 
     ( (SELECT j2.* FROM jobs AS j2 LEFT JOIN zipcode AS z2 ON z2.zipcode_id=j2.zipcode_id WHERE 1 AND source='manager' 
       ORDER BY postdate DESC LIMIT 30, 1000000) ORDER BY postdate DESC) 
s2)

Answer 2

如果你實際使用SELECT *那麼zipcode_id列是兩個Jobs表和Zipcode表。如錯誤消息所示，您不能使用與您擁有相同名稱的兩列。因爲你使用的是子查詢，如果你引用了重複的列名，那麼SQL引擎就無法理解你的意思了。例如，下面的SQL應該返回什麼？

SELECT num FROM (SELECT 1 AS num, 2 AS num) AS SQ 

使用SELECT *是在任何情況下一個非常糟糕的做法。