为什么我的查询不起作用. 我写了一个查询来识别缺席人数和在场人数

Question

select [First Name],[Last Name],Class,
(
select count([Date]) from Attendence where [Roll Number] ='1' and [Student Status] ='P' 
) as [No Of Present],
(
select count([Date]) from Attendence where [Roll Number] ='1' and [Student Status] ='A'
)as [No Of Absent]
from Attendence
where [Date] between '2018-09-1' and '2018-09-30'

这是我为我的软件编写的，但它没有给出正确的结果。我想计算学生的在场和缺席数量。

Answer 1

使用条件聚合：

select [First Name],[Last Name],Class from
count(case [Student Status] ='P' then 1 end) as [No Of Present], 
count(case [Student Status] ='A'  then 1 end)as [No Of Absent] 
from Attendence where [Date] between '2018-09-1' and '2018-09-30' and [Roll Number] ='1'
group by [First Name],[Last Name],Class

Answer 2

你的子查询与外部查询不同，它应该是相关的：

但是，您可以使用case表达式而不是两个子查询在单个select语句中执行此操作：

select [First Name], [Last Name], Class, 
sum(case when [Student Status] = 'P' then 1 else 0 end) as [No Of Present], 
sum(case when [Student Status] = 'A' then 1 else 0 end) as [No Of Absent] 
from Attendence at
where [Date] between '2018-09-1' and '2018-09-30' and [Roll Number] = '1'
group by [First Name], [Last Name], Class;

Answer 3

我认为您的查询可能有效，但您忘记使用不同。请找到以下更正的查询：

select distinct [First Name],[Last Name],Class,
(
select count([Date]) from Attendence where [Roll Number] ='1' and [Student Status] ='P' 
) as [No Of Present],
(
select count([Date]) from Attendence where [Roll Number] ='1' and [Student Status] ='A'
)as [No Of Absent]
from Attendence
where [Date] between '2018-09-1' and '2018-09-30'
group by [First Name],[Last Name],Class