Question

我有两张桌子：

CREATE TABLE Students
(
    Id int NOT NULL IDENTITY (1,1),
    FirstName nvarchar(50) NOT NULL,
    LastName nvarchar(50) NOT NULL,
    BirthDate datetime NOT NULL,
    IndexNumber nvarchar(20) NOT NULL,
    ParentName nvarchar(50) NULL,
    Gender char(1) NULL,
    CONSTRAINT PK_Students PRIMARY KEY (Id)
)

CREATE TABLE StudentComments
(
    Id int NOT NULL IDENTITY (1,1),
    Comment text NOT NULL,
    StudentId int NOT NULL,
    InsertAt datetime NOT NULL,
    IsPublished bit NOT NULL,
    CONSTRAINT PK_StudentComments PRIMARY KEY (Id),
    CONSTRAINT FK_StudentComments_Students FOREIGN KEY (StudentId) REFERENCES Students (Id)
)

我的任务是选择Students.Id，Students.FirstName，Students.LastName，StudentComments.Comment，StudentComments.InsertAt，以获取每个学生的最新发表评论。

我需要通过只使用一个select命令（而不是嵌套查询）来完成此操作。

我的尝试查询：

 SELECT s.Id,
       (s.FirstName + ' ' + s.LastName) AS Name,
       sc.Comment,
       sc.InsertAt,
       MAX(sc.InsertAt) AS NewestDate
 FROM Students AS s
 INNER JOIN StudentComments sc
  ON s.Id = sc.StudentsId AND sc.IsPublished = 1 
 GROUP BY /* Here is problem - I'm Begginer */
  HAVING sc.InsertAt = MAX(scInsertAt);

Answer 1

尝试以下解决方案之一：

SELECT  src.Id, src.FirstName, src.LastName, src.Comment, src.InsertAt
FROM 
(
    SELECT  s.Id, s.FirstName, s.LastName, sc.Comment, sc.InsertAt,
            ROW_NUMBER() OVER(PARTITION BY sc.StudentId ORDER BY sc.InsertAt DESC) RowNum
    FROM    dbo.Students s INNER JOIN dbo.StudentComments sc ON s.Id = sc.StudentId
    --WHERE sc.IsPublished = 1
) src
WHERE   src.RowNum = 1;

或

SELECT  s.Id, s.FirstName, s.LastName, lc.Comment, lc.InsertAt
FROM    dbo.Students s 
CROSS APPLY (
    SELECT  TOP(1) sc.Comment, sc.InsertAt
    FROM    dbo.StudentComments sc 
    WHERE   s.Id = sc.StudentId
    --AND       sc.IsPublished = 1
    ORDER BY sc.InsertAt DESC
) lc; -- Last comment

Answer 2

你需要一个子查询：

SELECT Students.Id, Students.FirstName, Students.LastName,
       (SELECT TOP 1 Comment from StudentComments WHERE StudentID = Students.ID ORDER BY InsertAt DESC) as Comment,
        (SELECT TOP 1 InsertAt from StudentComments WHERE StudentID = Students.ID ORDER BY InsertAt DESC) as InsertAt
FROM Students

如何选择学生创建的上次发表的评论？

2 个答案: