如何以不同的条件选择相同的字段两次并将结果显示为单独的字段

Posted 2023-05-08

【中文标题】如何以不同的条件选择相同的字段两次并将结果显示为单独的字段

【英文标题】：How to select the same field twice with different conditions and display results as separate fields

【发布时间】：2017-01-24 19:30:35

【问题描述】：

我想在 C# 中创建一个 Table Adapter，如下所示。

我希望在一个表中两次使用不同条件下的同一个字段，并且它必须显示为两个不同的字段。

这是我的源表：

从AttendanceReg 表中，我需要Count 基于AttendStatus 的每个学生的记录，以根据course and month 找到他们在Absent and present 的天数。

示例查询：

Select  AttendanceReg.StudentID
      , AttendanceReg.Studname
      , AttendanceReg.StudSex
      , AttendanceReg.StudCourse
      , Count(AttendanceReg.AttendStatus) As Total_Present
      , DatePart('m', AttendanceReg.DateOfAttendance) As Month
From    (AttendanceReg
         Inner Join LocalTable
            On AttendanceReg.StudCourse = LocalTable.AttendCourse
               And DatePart('M', AttendanceReg.DateOfAttendance) = LocalTable.AttentMonth
        )
Where   (AttendanceReg.AttendStatus = 'Present')
Group By AttendanceReg.StudentID
      , AttendanceReg.Studname
      , AttendanceReg.StudSex
      , AttendanceReg.StudCourse
      , DatePart('m', AttendanceReg.DateOfAttendance)
Order By DatePart('m', AttendanceReg.DateOfAttendance) Desc

查询结果和期望的结果（第一张图片显示了我得到的结果，第二张图片显示了我真正想要的结果）：

我需要做什么？

注意：我需要使用LocalTable，我不能传入参数来指定Course和Month

【参考方案1】：

 SELECT A.StudentID, A.Studname, A.StudSex, A.StudCourse, 
SUM(CASE WHEN A.AttendStatus = 'Present' THEN 1 ELSE 0 END) AS TotalPresent,
SUM(CASE WHEN A.AttendStatus = 'Absent' THEN 1 ELSE 0 END) AS TotalAbsent
FROM AttendanceReg A 
INNER JOIN LocalTable L ON A.StudCourse = L.AttendCourse AND DatePart('M', A.DateOfAttendance) = L.AttentMonth)

GROUP BY A.StudentID, A.Studname, A.StudSex, A.StudCourse

【讨论】：

执行时出现错误..无法解析查询文本。查询无法在图表和标准窗格中以图形方式表示。

我正在使用 Microsoft Access Database 2010，我认为 access 数据库不支持“CASE WHEN”

SELECT StudentID,Studname,StudSex,StudCourse, SUM(IIf(AttendStatus = 'Present', 1,0)) AS TotalPresent, SUM(IIf(AttendStatus = 'Absent', 1,0)) AS TotalAbsent FROM AttendanceReg GROUP BY StudentID,Studname,StudSex,StudCourse 这几乎可以工作......现在我如何使用 where 条件来查询？

可以在group by子句前添加where查询

【参考方案2】：

我终于解决了这个问题，感谢@Ikram Turgunbaev :)

这是代码

SELECT AttendanceReg.StudentID,AttendanceReg.Studname, AttendanceReg.StudSex,
AttendanceReg.StudCourse,
SUM(IIf( AttendanceReg.AttendStatus = 'Present', 1,0)) AS TotalPresent,
SUM(IIf( AttendanceReg.AttendStatus = 'Absent', 1,0)) AS TotalAbsent, 
DatePart('m', AttendanceReg.DateOfAttendance) AS MYMONTH

FROM (AttendanceReg
    INNER JOIN LocalTable ON
    AttendanceReg.StudCourse = LocalTable.AttendCourse 
    AND 
    DatePart('m', AttendanceReg.DateOfAttendance) = LocalTable.AttentMonth)

GROUP BY AttendanceReg.StudentID, AttendanceReg.Studname,
    AttendanceReg.StudSex, AttendanceReg.StudCourse,
    DatePart('m', AttendanceReg.DateOfAttendance)

ORDER BY DatePart('m', AttendanceReg.DateOfAttendance) DESC

【参考方案3】：

同样的事情也可以使用 Switch 来完成 :)

SELECT EmpAttendance.EmpID, EmpAttendance.Empname, EmpAttendance.EmpSex, EmpAttendance.EmpDepartment, EmpProfileTBL.Salary, 
SUM(SWITCH(EmpAttendance.AttendStatus = 'Present', 1, EmpAttendance.AttendStatus = 'Absent', 0, EmpAttendance.AttendStatus = 'Half-Day', 0)) AS TotalPresent,
SUM(SWITCH(EmpAttendance.AttendStatus = 'Absent', 1, EmpAttendance.AttendStatus = 'Present', 0, EmpAttendance.AttendStatus = 'Half-Day', 0)) AS TotalAbsent,
SUM(SWITCH(EmpAttendance.AttendStatus = 'Half-Day', 1, EmpAttendance.AttendStatus = 'Present', 0, EmpAttendance.AttendStatus = 'Absent', 0)) AS HalfDay,

DatePart('m', EmpAttendance.DateOfAttendance) AS CurMonth,
((EmpProfileTBL.Salary * TotalPresent) - (EmpProfileTBL.Salary * TotalAbsent) 
                         + ((EmpProfileTBL.Salary / 2) * HalfDay)) AS FinalSalary
FROM ((EmpAttendance INNER JOIN LocalVariableTable
ON EmpAttendance.EmpDepartment = LocalVariableTable.TempDepartment AND
DatePart('m', EmpAttendance.DateOfAttendance) = LocalVariableTable.TempMonth
 AND DatePart('yyyy', EmpAttendance.DateOfAttendance) = LocalVariableTable.TempYear)
INNER JOIN
EmpProfileTBL ON EmpAttendance.EmpID = EmpProfileTBL.EmpID)

GROUP BY EmpAttendance.EmpID, EmpAttendance.Empname, EmpAttendance.EmpSex,
EmpAttendance.EmpDepartment, DatePart('m', EmpAttendance.DateOfAttendance), 
EmpProfileTBL.Salary
ORDER BY DatePart('m', EmpAttendance.DateOfAttendance) DESC