Employees Table: Employee
+ --------- + --------------- +
| Column the Name | Type |
+ --------------- + + ---------
| employee_id | int |
| team_id | int |
+ --------------- + --------- +
employee_id field is the primary key of this table, for each row in the table contains each employee's ID and the team to which they belong.
Write a SQL query to obtain each employee where the total number of teams.
The order of query results without specific requirements.
Query Result Format examples are as follows:
Employee Table:
+-------------+------------+
| employee_id | team_id |
+-------------+------------+
| 1 | 8 |
| 2 | 8 |
| 3 | 8 |
| 4 | 7 |
| 5 | 9 |
| 6 | 9 |
+-------------+------------+
Result table:
+-------------+------------+
| employee_id | team_size |
+-------------+------------+
| 1 | 3 |
| 2 | 3 |
| 3 | 3 |
| 4 | 1 |
| 5 | 2 |
| 6 | 2 |
+ ------------ + ------------- +
ID is 1, 2, as a member of the team staff is team_id 8,
ID of 4 employees are team_id for the team of 7 members,
ID 5, 6 employees as members of the team are team_id 9.
Source: stay button (LeetCode)
link: https: //leetcode-cn.com/problems/find-the-team-size
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Moderation: to write a SQL query to obtain each employee where the total number of teams.
Thoughts: Query behind team id id summation.
Problem solving:
select employee_id,
(select count(*) from Employee e2
where e1.team_id = e2.team_id) as team_size
from Employee e1
-- 练习
select employee_id (select count(*) from Employee e2
where e1.team_id = e2.team_id)as team_size
from Employee e1;Method two: left join
select e1.employee_id, count(*) team_size
from employee e1 left join employee e2
on e1.team_id = e2.team_id
group by e1.employee_id;
-- 通过left join
select e1.employee_id,count(*) team_size
from employee e1 left join employee e2
on e1.team_id = e2.team_id
group by e1.employee_id;Knowledge points:
