在嵌套子查询中引用表

Question

如何在 WHERE 子句中引用“最外层”表（别名为“cmp”）中的 ID_comp 字段：

SELECT
      (SELECT COUNT(*) cnt
       FROM 
           (SELECT trp.trip_no
        FROM Trip trp
        WHERE trp.ID_comp = cmp.ID_comp
        GROUP BY trp.trip_no) foo
      ) flights
FROM Company cmp

上面的代码给了我一个错误："Unknown column 'cmp.ID_comp' in 'where clause'"?

Answer 1

我想你正在寻找这个：

SELECT 
   C.ID_comp,
   COUNT(T.ID_comp) trip_count
FROM company C
  LEFT JOIN Trip T
    ON C.ID_comp = T.ID_comp
GROUP BY C.ID_comp