mysql - 在进一步的子查询中引用（基于子查询的）别名

Question

我可以在 where 子句的第二个子查询中引用从 FROM 子句中的子查询创建的别名吗？

Select x FROM 
((SELECT x,y FROM A) UNION (SELECT x,y FROM B) UNION (SELECT x,y FROM C)) AS newAlias 
WHERE y IN (SELECT MAX(y) FROM newAlias)

Answer 1

不，答案是否定的。NewAlias必须重复子查询。

可以使用 CTE（通用表表达式），但 MySQL 尚未实现它们：

WITH newAlias AS
  (SELECT x,y FROM A UNION SELECT x,y FROM B UNION SELECT x,y FROM C)
SELECT x 
FROM newAlias 
WHERE y = (SELECT MAX(y) FROM newAlias) ;

在 MySQL 中，您必须重复子查询：

SELECT x 
FROM 
  (SELECT x,y FROM A UNION SELECT x,y FROM B UNION SELECT x,y FROM C) 
  AS newAlias 
WHERE y = 
      ( SELECT MAX(y) 
        FROM
          (SELECT x,y FROM A UNION SELECT x,y FROM B UNION SELECT x,y FROM C) 
          AS newAlias  
      ) ;

当然，您通常可以重新编写查询。一种方法：

SELECT x 
FROM 
  (SELECT x,y FROM A UNION SELECT x,y FROM B UNION SELECT x,y FROM C) 
  AS newAlias 
WHERE y = GREATEST( (SELECT MAX(y) FROM A)
                  , (SELECT MAX(y) FROM B)
                  , (SELECT MAX(y) FROM C)
                  ) ;