[英]Select all rows from one table and one specific value for each row from another table
[英]Select count from another table to each row in result rows
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
我想获取班级列表和每个班级 - 属于每个班级的学生人数。 我怎样才能做到这一点?
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
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]
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
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
无需添加group by
子句,您可以执行以下操作:
创建一个函数来计算学生人数:
go
CREATE FUNCTION [dbo].GetStudentsCountByClass(@classId int) RETURNS INT
AS BEGIN
declare @count as int
select @count = count(*) from STUDENTS
where ClassId = @classId
RETURN @count
END
然后在您的select
语句中使用它
SELECT * , dbo.GetStudentsCountByClass(ClassId) AS StudentsCount
FROM Classes
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