.net - 选择参数中的 TSQL 计数

Question

给定以下查询，它将计算给定入学（科目）姓氏史密斯的学生人数

SELECT 
  e.EnrolmentId,
  COUNT((CASE WHEN s.Surname LIKE 'Smith' THEN 1 ELSE NULL END)) AS CountIf
FROM 
  Enrolment e
INNER JOIN StudentEnrolment se
  ON e.EnrolmentId = se.EnrolmentId
INNER JOIN Student s
  ON se.StudentId = s.StudentId
GROUP BY
  e.EnrolmentId

是否可以将“史密斯”作为参数，像这样

COUNT((CASE WHEN s.Surname LIKE @Surname THEN 1 ELSE NULL END)) AS CountIf

Answer 1

如果您将 COUNT 更改为 SUM 并将 ELSE 部分更改为 0 而不是 NULL，它应该可以工作，不是吗？

SELECT 
  e.EnrolmentId,
  SUM((CASE WHEN s.Surname LIKE 'Smith' THEN 1 ELSE 0 END)) AS CountIf
FROM 
  Enrolment e
INNER JOIN StudentEnrolment se
  ON e.EnrolmentId = se.EnrolmentId
INNER JOIN Student s
  ON se.StudentId = s.StudentId
GROUP BY
  e.EnrolmentId

例子：

declare @t as varchar(20)
select @t='ricardo'

declare @ta as table
(
    name varchar(30)
)

insert into @ta
values
('juan'),
('ricardo'),
('pablo')


select
SUM((CASE WHEN name LIKE @t THEN 1 ELSE 0 END)) AS CountIf
from @ta

输出：

CountIf
-----------
1