力扣高频SQL 50题（基础版）第十七题

1075. 项目员工 I

题目说明

项目表 Project：

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| project_id  | int     |
| employee_id | int     |
+-------------+---------+
主键为 (project_id, employee_id)。
employee_id 是员工表 Employee 表的外键。
这张表的每一行表示 employee_id 的员工正在 project_id 的项目上工作。

员工表 Employee：

±-----------------±--------+

| Column Name | Type |

±-----------------±--------+

| employee_id | int |

| name | varchar |

| experience_years | int |

±-----------------±--------+

主键是 employee_id。数据保证 experience_years 非空。

这张表的每一行包含一个员工的信息。

请写一个 SQL 语句，查询每一个项目中员工的 平均 工作年限，精确到小数点后两位。

以 任意 顺序返回结果表。

思路分析

实现过程

准备数据

Create table If Not Exists Project (project_id int, employee_id int)
Create table If Not Exists Employee (employee_id int, name varchar(10), experience_years int)
Truncate table Project
insert into Project (project_id, employee_id) values ('1', '1')
insert into Project (project_id, employee_id) values ('1', '2')
insert into Project (project_id, employee_id) values ('1', '3')
insert into Project (project_id, employee_id) values ('2', '1')
insert into Project (project_id, employee_id) values ('2', '4')
Truncate table Employee
insert into Employee (employee_id, name, experience_years) values ('1', 'Khaled', '3')
insert into Employee (employee_id, name, experience_years) values ('2', 'Ali', '2')
insert into Employee (employee_id, name, experience_years) values ('3', 'John', '1')
insert into Employee (employee_id, name, experience_years) values ('4', 'Doe', '2')

实现方式

with t1 as (select e.employee_id,e.experience_years,p.project_id from Employee e left join Project p
    on p.employee_id=e.employee_id)
select project_id,round(avg(experience_years),2) average_years from t1 where project_id is not null
                                                                       group by project_id;

结果截图