[我正在尝试在SQL中使用带有内部联接的分组方式，但它会产生错误

Question

您好，这是有效的sql语句，并且在SQL Server女士中正常工作。

select classes.Id, classes.className, classes.Level, classes.timing
from classes
inner join StudentClasses on classes.Id = StudentClasses.classId;

它产生的结果很好。

13  Trigonometry    Beginar     Morning
13  Trigonometry    Beginar     Morning
13  Trigonometry    Beginar     Morning
14  Maths           Advanced    Evening
15  Trigonometry    Advanced    Evening

现在我想按classes.Id对它们进行分组，但是当我尝试按以下方式使用分组依据时，它会产生错误：

select classes.Id, classes.className, classes.Level, classes.timing
from classes
inner join StudentClasses on classes.Id = StudentClasses.classId
group by classes.Id 
;

这是错误：

Msg 8120, Level 16, State 1, Line 1
Column 'classes.className' is invalid in the select list because it is not contained in either an 
aggregate function or the GROUP BY clause.

Answer 1

您必须按所选的所有列分组，但汇总列除外。尝试以下。

如果有重复项，则可以使用distinct。

select classes.Id, classes.className, classes.Level, classes.timing
from classes
inner join StudentClasses on classes.Id = StudentClasses.classId
group by classes.Id, classes.className, classes.Level, classes.timing 
;