错误：列无效，因为它不包含在聚合函数或GROUP BY中

Question

我有这样的代码，工作正常：

select count (CAST(faculty AS NVARCHAR(MAX))) as num
from student
group by CAST(faculty AS NVARCHAR(MAX))

faculty是一个文字。但是我需要计算一列中的教师数量，所以尝试了这个代码，它给出了错误： Column＆＃39; student.faculty＆＃39;在选择列表中无效，因为它不包含在聚合函数或GROUP BY子句中。

select faculty, count (CAST(faculty AS NVARCHAR(MAX))) as num
from student
group by CAST(faculty AS NVARCHAR(MAX))

我无法理解，问题在哪里，因为同样的方法适用于数字。例如这一个：

select id, count(id)  
from student
group by id

我做错了什么？

Answer 1

您收到错误，因为您没有按照您选择的相同分组 CAST(faculty AS NVARCHAR(MAX))不等于faculty。为什么不使用：

select faculty, count (*) as num
from student
group by faculty

Answer 2

试试这个

select count(CAST(faculty AS NVARCHAR(MAX))) as num ,
             CAST(faculty AS NVARCHAR(MAX)) facultys
from student
group by facultys