从另一个表中选择计数到结果行中的每一行

Here are the tables:

以下是表格:

CREATE TABLE [dbo].[Classes](
    [ClassId] [int] NOT NULL,
    [ClassName] [nvarchar](50) NOT NULL,
 CONSTRAINT [PK_Classes] PRIMARY KEY CLUSTERED 
(
    [ClassId] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[Students](
    [StudentId] [int] NOT NULL,
    [ClassId] [int] NOT NULL,
 CONSTRAINT [PK_Students] PRIMARY KEY CLUSTERED 
(
    [StudentId] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
ALTER TABLE [dbo].[Students]  WITH CHECK ADD  CONSTRAINT [FK_Students_Classes] FOREIGN KEY([ClassId])
REFERENCES [dbo].[Classes] ([ClassId])
GO
ALTER TABLE [dbo].[Students] CHECK CONSTRAINT [FK_Students_Classes]
GO

I want to get list of class, and each class - the number of student which belong to each class. How can I do this?

我想获得课程列表,以及每个班级 - 每个班级的学生人数。我怎样才能做到这一点?

4 个解决方案

#1

You need to do this -

你需要这样做 -

SELECT C.ClassId, C.ClassName, count(S.StudentId) AS studentCount
FROM CLASSES C LEFT JOIN STUDENTS S ON (C.ClassId=S.ClassId)
GROUP BY C.ClassId, C.ClassName

#2

You mean something like this?

你的意思是这样的?

SELECT C.[ClassName], COUNT(*) AS 'Number of Students'
FROM [dbo].[Classes] AS C
    INNER JOIN [dbo].[Students] AS S ON S.[ClassId] = C.[ClassId]
GROUP BY C.[ClassName]

#3

 select c.ClassId,C.ClassName,COUNT(*) [Number of students]
 from Classes C,Students S
 where c.ClassId=S.ClassId
 group by C.ClassId,C.ClassName

#4

SELECT class.ClassId, count(student .StudentId) AS studentCount
FROM dbo.CLASSES class LEFT JOIN dbo.STUDENTS student ON (class.ClassId=student.ClassId)
GROUP BY class.ClassId

#1