带有示例架构的 SQL 连接

Question

我有以下架构：

*Project*
ID(PK)  NAME
1       Alpha
2       Bravo
3       Charlie
4       Delta

*Employee*
ID(PK)  NAME
1       Kevin
2       Mike
3       Eric
4       Ira
5       Peter

*Project Assignment*
ID(PK)  ProjectID(FK)   EmployeeID(FK)
1       1               1
2       1               2
3       2               2
4       2               3
5       3               3
6       3               4
7       1               3

我想编写查询：

1) 显示所有分配的项目及其团队成员。

期望的输出：

Project TeamMembers
Alpha   Eric
Alpha   Kevin
Alpha   Mike
Bravo   Eric
Bravo   Mike
Charlie Eric
Charlie Ira

2) 仅显示 Alpha 项目及其团队成员，并按降序显示团队成员的姓名。

期望的输出：

ID      TeamMember
Alpha   Mike
Alpha   Kevin
Alpha   Eric

最简单的查询会是什么样子？

附加问题：

查询每个项目的团队成员数量是多少？

期望的结果：

+-------------+-------------+
| ProjectName | NumMembers |
+-------------+-------------+
| Alpha       | 3           |
| Bravo       | 2           |
| Charlie     | 2           |
+-------------+-------------+

Answer 1

create table Project (ID int,  NAME varchar(50));

insert into Project values
(1, 'Alpha'),
(2, 'Bravo'),
(3, 'Charlie'),
(4, 'Delta');

create table Employee (ID int,  NAME varchar(50));

insert into Employee values
(1, 'Kevin'), 
(2, 'Mike'),
(3, 'Eric'),
(4, 'Ira'),
(5, 'Peter');

create table Project_Assignment (ID int,  ProjectID int,  EmployeeID int);

insert into Project_Assignment values
(1, 1,  1),
(2, 1,  2),
(3, 2,  2),
(4, 2,  3),
(5, 3,  3),
(6, 3,  4),
(7, 1,  3);

解决方案#1：

您需要将Project_Assignment 表加入Project 和Employee 表以分别获取您的两列：

select p.name as projectname,
       e.name as TeamMembers   
from project_assignment pa
join project p on pa.projectid = p.id
join employee e on pa.employeeid = e.id
order by projectname, TeamMembers desc;   

结果：

+-------------+-------------+
| projectname | TeamMembers |
+-------------+-------------+
| Alpha       | Mike        |
| Alpha       | Kevin       |
| Alpha       | Eric        |
| Bravo       | Mike        |
| Bravo       | Eric        |
| Charlie     | Ira         |
| Charlie     | Eric        |
+-------------+-------------+

SQL Fiddle Demo

解决方案#2：

在上述查询中添加where 子句以仅过滤Alpha 项目：

select p.name as projectname,
       e.name as TeamMembers   
from project_assignment pa
join project p on pa.projectid = p.id
join employee e on pa.employeeid = e.id
where p.name = 'Alpha'
order by projectname, TeamMembers desc;

结果：

+-------------+-------------+
| projectname | TeamMembers |
+-------------+-------------+
| Alpha       | Mike        |
| Alpha       | Kevin       |
| Alpha       | Eric        |
+-------------+-------------+

SQL Fiddle Demo 2