引用嵌套子查询中的表

Question

如何在＆＃34;最外层＆＃34;中引用ID_comp字段？ WHERE子句中的表（别名为＆＃34; cmp＆＃34;）：

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

我认为你正在寻找这个：

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