SQL：从不同子查询中的多个 JOINS 中选择答案

【问题标题】：SQL: Select from multiple JOINS in different subqueriesSQL：从不同子查询中的多个 JOINS 中选择
【发布时间】：2020-08-20 06:30:44
【问题描述】：

我有两个表 Employees 和 Assignments，其内容如下图所示（分别输出 A 和 B）。 Employees 中的每个员工最多有1 名项目负责人 (Assignment.A_Leader = 1)，最多 1 名。 1 个项目控制器 (Assignment.A_Controller = 1) 分配。我想获取所有员工的列表，以及他们是领导者或控制者的项目。如果员工没有此类分配，则应出现NULL。我设法列出领导或项目任务。但是，我无法按照here 的建议将两者放在适当的列表中（参见尝试 E）。为了获得所需的决赛桌，我必须添加什么？

下面代码的字母 A-E 对应于图像中输出的字母 A-E。

我使用的是 Microsoft SQL Server 2017。

-- A: all employees
SELECT * FROM Employee

-- B: all assignments
SELECT * FROM Assignment

-- C: employees with their projects as LEADERS
SELECT E.E_FirstName as First, E.E_LastName as Last, L.A_ProjectID as Leader
FROM Employee E LEFT JOIN
(SELECT A.E_ID, A_ProjectID FROM Assignment A WHERE A.A_Leader=1) L
ON E.E_ID = L.E_ID

-- D: employees with their projects as CONTROLLERS
SELECT E.E_FirstName as First, E.E_LastName as Last, C.A_ProjectID as Controller
FROM Employee E LEFT JOIN
(SELECT A.E_ID, A_ProjectID FROM Assignment A WHERE A.A_Controller=1) C
ON E.E_ID = C.E_ID

-- E: employees with their projects as LEADERS or CONTROLLERS
SELECT X.First, X.Last, X.Leader, Y.Controller FROM
(SELECT E.E_FirstName as First, E.E_LastName as Last, L.A_ProjectID as Leader
FROM Employee E LEFT JOIN
(SELECT A.E_ID, A_ProjectID FROM Assignment A WHERE A.A_Leader=1) L
ON E.E_ID = L.E_ID) as X,
(SELECT E.E_FirstName as First, E.E_LastName as Last, C.A_ProjectID as Controller
FROM Employee E LEFT JOIN
(SELECT A.E_ID, A_ProjectID FROM Assignment A WHERE A.A_Controller=1) C
ON E.E_ID = C.E_ID) as Y

【问题讨论】：

标签： sql-server tsql join select ssms-2017

【解决方案1】：

您可以只left joinassignments 表两次：

select
    e.e_first_name,
    e.e_last_name,
    al.a_project_id leader,
    ac.a_project_id controller
from employee e
left join assignments al on al.e_id = e.e_id and al.a_leader     = 1
left join assignments ac on ac.e_id = e.e_id and ac.a_controller = 1

【讨论】：