1、題目描述
Write an SQL query that reports the most experienced employees in each project. In case of a tie, report all employees with the maximum number of experience years.
The query result format is in the following example:
Project table:
| project_id | employee_id |
|---|---|
| 1 | 1 |
| 1 | 2 |
| 1 | 3 |
| 2 | 1 |
| 2 | 4 |
Employee table:
| employee_id | name | experience_years |
|---|---|---|
| 1 | Khaled | 3 |
| 2 | Ali | 2 |
| 3 | John | 3 |
| 4 | Doe | 2 |
Result table:
| project_id | employee_id |
|---|---|
| 1 | 1 |
| 1 | 3 |
| 2 | 1 |
Both employees with id 1 and 3 have the most experience among the employees of the first project. For the second project, the employee with id 1 has the most experience.
來源：力扣（LeetCode）
2、解題思路
主要是出現多個最多經驗的，重複數據處理
1# 首選2表聯查，找出每個project_id最大經驗值
select project_id,e.employee_id,max(experience_years) as `years`
from Project p left join Employee e
on e.employee_id=p.employee_id
group by project_id
2# 然後，Project、Employee還有表，3表聯查，條件是experience_years等於上述最大經驗值
3、提交記錄
select p0.project_id,p0.employee_id
from Project p0 left join Employee e0
on p0.employee_id=e0.employee_id
left join
(select project_id,e.employee_id,max(experience_years) as `years`
from Project p left join Employee e
on e.employee_id=p.employee_id
group by project_id) max1
on p0.project_id=max1.project_id
where e0.experience_years=max1.`years`
1190ms