与“按分组计数记录”相关的问题

Question

我有一个StudentId(int),Subject(varchar(50)),Marks(int),IsPass(int),ExamDate(Datetime)

的数据库表

对于不同日期的特定学生，表格可以为同一主题提供多条记录。

我写了以下查询：

select StudentId, Count(IsPass) 
from ExamEntry
where IsPass =1  group by StudentId

但不希望查询中的Where条件：

这可能是这样的：

Select StudentId, case when IsPass = 1 then count(IsPass) end 
from ExamEntry 
group by studentId

但是它显示了针对特定studentId的一条记录

我如何实现目标？

Answer 1

答案是如此简单，没有人看到它:-)

SELECT StudentId, SUM(IsPass) 
FROM ExamEntry GROUP BY StudentId

Answer 2

SELECT StudentId,
       COUNT(CASE WHEN IsPass = 1 THEN 'X' END) AS NumberOfPasses
FROM   ExamEntry
GROUP  BY StudentId  

COUNT仅计算NOT NULL个值，CASE具有隐式ELSE NULL

Answer 3

也许你需要这样的东西？

SELECT StudentId, SUM(CASE WHEN IsPass = 1 THEN 1 ELSE 0 END) AS Number_Of_Passed_Exams
FROM ExamEntry
GROUP BY StudentId

Answer 4

如果你不想使用where子句，那么Having子句是最简单的替代。

试试这个。

select StudentId, count(IsPass)AS PassedCount
from ExamEntry
group by StudentId,IsPass
having IsPass= 1