Question

我正在尝试运行一个查询，该查询返回在名为Hospital的表中出现超过3次的ID列表。到目前为止，我已经想出了这个：

SELECT doctorID
FROM Hospital 
WHERE doctorID IN

(SELECT temp.doctorID, count(temp.doctorID) as frequency
FROM Hospital as temp
GROUP BY temp.doctorID
HAVING frequency >3);

但出于某种原因，我在运行时遇到错误

Code: 1241. Operand should contain 1 column(s)

当我环顾四周时似乎是一个流行的错误，但我找不到适用于我的现有解决方案。当我在括号中运行查询时，它可以正常工作。那么为什么它不能作为子查询工作呢？

Answer 1

从子查询中删除第二列，将count()移到HAVING子句中：

SELECT doctorID
FROM Hospital 
WHERE doctorID IN

(SELECT temp.doctorID
FROM Hospital AS temp
GROUP BY temp.doctorID
HAVING count(temp.doctorID) > 3);

如果没有子查询，您可以使结果更简单：

SELECT doctorID
FROM   Hospital
GROUP  BY doctorID
HAVING count(doctorID) > 3;

Answer 2

SELECT doctorID
FROM Hospital 
WHERE doctorID IN

(SELECT temp.doctorID 
FROM Hospital as temp
GROUP BY temp.doctorID
HAVING count(temp.doctorID)>3);

Answer 3

您的子查询返回2列。从SELECT列表中删除COUNT列，如下所示：

SELECT doctorID
FROM Hospital 
WHERE doctorID IN
   (SELECT temp.doctorID
   FROM Hospital as temp
   GROUP BY temp.doctorID
   HAVING count(temp.doctorID) >3);

子查询没有给出输出

3 个答案: