如何组合 3 个 Join 的结果？

Question

我正在尝试为每位员工创建一个表，该表将为每位员工提供在家/不在/在家工作的信息。为此，我正在编写一个使用 JOINS 获取记录的 SQL 查询。但是，我正在编写 3 个分别返回数据的 Join 语句。如何编写一个查询，在一个 (SELECT) 查询中返回 3 个后续连接的结果？以下是 Join 语句：

Select EmployeeId, Employees.Name, COUNT(*) [Absent]
From Attendances
Left Join Employees
On Employees.EmployeeId = Attendances.EmpID And Attendances.IsAbsent = 1 and Employees.Name = 'John'
Where CAST(Attendances.InDateTime as Date) >= '2016-06-19 10:00:00.000' 
Group By EmployeeId, Employees.Name

select Employees.Name, COUNT(*) [Present]
from Employees
Inner Join Attendances
On Employees.UserId = Attendances.EmpID and Attendances.IsAbsent = 0
Where CAST(Attendances.InDateTime as Date) >= '2017-06-19 10:00:00.000' and Employees.Name='John'
Group By Employees.Name

select Employees.Name, COUNT(*) [WorkFromHome]
from Employees
Inner Join Attendances
On Employees.UserId = Attendances.EmpID and Attendances.IsWorkFromHome = 1
Where CAST(Attendances.InDateTime as Date) >= '2017-06-19 10:00:00.000' and Employees.Name='John'
Group By Employees.Name

我得到的当前输出是： 姓名|现在，姓名|缺席，姓名|在家工作 但我希望它为： 姓名|现在|缺席|在家工作

Answer 1

使用Subqueries 获得您想要的输出：

SELECT T1.ID,
       T1.Name,
       T1.[Absent],
       T2.[Present],
       T3.[WorkFromHome]
FROM
(
select ID, Employees.Name, COUNT(*) [Absent]
from Employees
Inner Join Attendances
On Employees.UserId = Attendances.EmpID and Attendances.IsAbsent = 1
Where CAST(Attendances.InDateTime as Date) >= '2017-06-19 10:00:00.000' and Employees.Name='John'
Group By Employees.Name, ID
) T1 JOIN
(
select ID, Employees.Name, COUNT(*) [Present]
from Employees
Inner Join Attendances
On Employees.UserId = Attendances.EmpID and Attendances.IsAbsent = 0
Where CAST(Attendances.InDateTime as Date) >= '2017-06-19 10:00:00.000' and Employees.Name='John'
Group By Employees.Name, ID
) T2 ON T1.ID = T2.ID JOIN
(
select ID, Employees.Name, COUNT(*) [WorkFromHome]
from Employees
Inner Join Attendances
On Employees.UserId = Attendances.EmpID and Attendances.IsWorkFromHome = 1
Where CAST(Attendances.InDateTime as Date) >= '2017-06-19 10:00:00.000' and Employees.Name='John'
Group By Employees.Name, ID
) T3 ON T2.ID = T3.ID;

Answer 2

您可以使用 UNION 或 UNION ALL。例如

SELECT * FROM TABLE WHERE NAME = 'Mohit'
UNION ALL
SELECT * FROM TABLE WHERE NAME = 'Jhon'
UNION ALL
SELECT * FROM TABLE WHERE NAME = 'Jeni'

UNION ALL 将返回唯一记录。但是 UNION 将返回所有记录。

Answer 3

使用 UNION ALL 得到这样的组合数据

select Employees.Name, 'A' Attendance, COUNT(*) [Count]
from Employees
Inner Join Attendances
On Employees.UserId = Attendances.EmpID and Attendances.IsAbsent = 1
Where CAST(Attendances.InDateTime as Date) >= '2017-06-19 10:00:00.000' and 
Employees.Name='John'
Group By Employees.Name
UNION ALL
select Employees.Name, 'P' Attendance, COUNT(*) [Count]
from Employees
Inner Join Attendances
On Employees.UserId = Attendances.EmpID and Attendances.IsAbsent = 0
Where CAST(Attendances.InDateTime as Date) >= '2017-06-19 10:00:00.000' and 
Employees.Name='John'
Group By Employees.Name
UNION ALL
select Employees.Name, 'WFH' Attendance, COUNT(*) [Count]
from Employees
Inner Join Attendances
On Employees.UserId = Attendances.EmpID and Attendances.IsWorkFromHome = 1
Where CAST(Attendances.InDateTime as Date) >= '2017-06-19 10:00:00.000' and 
Employees.Name='John'
Group By Employees.Name