SQL子查询返回多个值

Question

我正在尝试返回所有舱位名称，这些舱位名称的容纳人数大于其舱位类型可以容纳的平均人数。

SELECT CNAME AS "CABIN NAME", NUMACCOM AS "Can Accomodate"
FROM cabinWH 
WHERE NUMACCOM > (SELECT AVG(NUMACCOM)  FROM cabinWH GROUP BY CTYPE)

错误：

  

子查询返回了多个值。当子查询遵循=，！=，<，<=，>，> =或将子查询用作表达式时，不允许这样做。

Answer 1

我认为您想考虑一种特定的类型，而不是按类型分组，这当然会给您多行，每行一种：

SELECT CNAME AS "CABIN NAME", NUMACCOM AS "Can Accomodate"
FROM cabinWH C1
WHERE NUMACCOM > (SELECT AVG(C2.NUMACCOM) FROM cabinWH C2 where C2.CTYPE = C1.CTYPE)

注意：如果要对整个结果集取平均值，则不需要分组。

Answer 2

您应该在子查询中放置where子句，以确保从中得到单个结果：

SELECT outerCabin.CNAME AS "CABIN NAME", outerCabin.NUMACCOM AS "Can Accomodate"
FROM cabinWH outerCabin
WHERE NUMACCOM >
(SELECT AVG(innerCabin.NUMACCOM)  FROM cabinWH innerCabin 
where outerCabin.CTYPE = innerCabin.CTYPE GROUP BY innerCabin.CTYPE)

尝试一下。