SQL- 带有聚合函数的子查询 (SQL Server)答案

【问题标题】：SQL- Subquery with aggregate functions (SQL Server)SQL- 带有聚合函数的子查询 (SQL Server)
【发布时间】：2013-04-09 09:18:02
【问题描述】：

我的设置是这样的：我有三张桌子-

学生（学生 ID、名字、姓氏等），
StudentSemesters（StudentID、SemID 等）和
学期（SemID，年份）

我的要求是获取每个学生的详细信息，但仅限于他们最后一个学期的详细信息。从逻辑上讲，这意味着年数最高的学期。我似乎无法正确查询。为简单起见，“年份”只是一个整数（例如 2000、1998）。以下是我一直坚持的当前查询：

SELECT dbo.Student.LastName + ' , ' + dbo.Student.FirstName AS Student, dbo.Student.Defence1Date, dbo.Student.Defence2Date, COUNT(StudentSemesters_1.SemID) 
           AS SemesterCount, dbo.Student.EntrySemester + ' - ' +
               (SELECT dbo.StudentSemesters.SemID
                FROM   dbo.StudentSemesters INNER JOIN
                               dbo.ListSemesters ON dbo.StudentSemesters.SemID = dbo.ListSemesters.SemID
                WHERE (dbo.Student.StudentCode = dbo.StudentSemesters.StudentCode)
                GROUP BY dbo.StudentSemesters.SemID, dbo.ListSemesters.Year
                HAVING (dbo.ListSemesters.Year = MAX(dbo.ListSemesters.Year))) AS Expr1
FROM  dbo.Student INNER JOIN
           dbo.StudentSemesters AS StudentSemesters_1 ON dbo.Student.StudentCode = StudentSemesters_1.StudentCode
GROUP BY dbo.Student.LastName, dbo.Student.FirstName, dbo.Student.Defence1Date, dbo.Student.Defence2Date, dbo.Student.EntrySemester, 
           dbo.Student.StudentCode

【问题讨论】：

标签： sql-server

【解决方案1】：

您可以这样做以仅获取每个学生最后一个学期的详细信息：

SELECT 
  s.LastName + ' , ' + s.FirstName   AS Student,  
  s.Defence1Date, 
  s.EntrySemester + ' - ' + s1.SemId AS Expr1
FROM  dbo.Student               AS s
INNER JOIN dbo.StudentSemesters AS S1 ON s.StudentCode = S1.StudentCode
INNER JOIN dbo.ListSemesters    AS lm ON lm.SemID      = s1.SemId
INNER JOIN 
(
    SELECT SemId, MAX(Year) AS MaxYear
    FROM dbo.StudentSemesters
    GROUP BY SemId
) AS s2 ON s2.semId = lm.SemId AND ls.Year = s2.MaxYear;

但是，如果您使用的是 SQL Server 2005+，则可以使用窗口函数来执行此操作：

WITH CTE
AS
(
    SELECT 
      s.LastName + ' , ' + s.FirstName   AS Student,  
      s.Defence1Date, 
      s.EntrySemester + ' - ' + s1.SemId AS Expr1,
      ROW_NUMBER() OVER(PARTITION BY s1.SemId
                        ORDER BY Year DESC) AS RowNumber
    FROM  dbo.Student               AS s
    INNER JOIN dbo.StudentSemesters AS S1 ON s.StudentCode = S1.StudentCode
    INNER JOIN dbo.ListSemesters    AS lm ON lm.SemID      = s1.SemId
) 
SELECT 
  Student,
  Defence1Date,
  Expr1,
FROM CTE
WHERE RN = 1;

但这不会得到SemesterCount，但你也许可以这样（这只是一个猜测）：

SELECT 
  s.LastName + ' , ' + s.FirstName   AS Student,  
  s.Defence1Date, 
  s1.SemCount,
  s.EntrySemester + ' - ' + s1.SemId AS Expr1
FROM  dbo.Student               AS s
INNER JOIN
(
   SELECT StudentCode, COUNT(SemId) AS SemCount
   FROM dbo.StudentSemesters 
   GROUP BY StudentCode
) AS S1 ON s.StudentCode = S1.StudentCode
INNER JOIN dbo.ListSemesters    AS lm ON lm.SemID      = s1.SemId
INNER JOIN 
(
    SELECT SemId, MAX(Year) AS MaxYear
    FROM dbo.StudentSemesters
    GROUP BY SemId
) AS s2 ON s2.semId = lm.SemId AND ls.Year = s2.MaxYear;

【讨论】：