1303. Find the Team Size

Description

Table: Employee

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| employee_id   | int     |
| team_id       | int     |
+---------------+---------+
employee_id is the primary key (column with unique values) for this table.
Each row of this table contains the ID of each employee and their respective team.

 

Write a solution to find the team size of each of the employees.

Return the result table in any order.

The result format is in the following example.

 

Example 1:

Input: 
Employee Table:
+-------------+------------+
| employee_id | team_id    |
+-------------+------------+
|     1       |     8      |
|     2       |     8      |
|     3       |     8      |
|     4       |     7      |
|     5       |     9      |
|     6       |     9      |
+-------------+------------+
Output: 
+-------------+------------+
| employee_id | team_size  |
+-------------+------------+
|     1       |     3      |
|     2       |     3      |
|     3       |     3      |
|     4       |     1      |
|     5       |     2      |
|     6       |     2      |
+-------------+------------+
Explanation: 
Employees with Id 1,2,3 are part of a team with team_id = 8.
Employee with Id 4 is part of a team with team_id = 7.
Employees with Id 5,6 are part of a team with team_id = 9.

Solutions

Solution 1: Group By + Equi-Join

We can first count the number of people in each team and record it in the T table. Then, we can use an equi-join to join the Employee table and the T table based on team_id, and obtain the total number of people in each team.

SQL Code
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# Write your MySQL query statement below
WITH
    T AS (
        SELECT team_id, COUNT(1) AS team_size
        FROM Employee
        GROUP BY 1
    )
SELECT employee_id, team_size
FROM
    Employee
    JOIN T USING (team_id);

Solution 2: Left Join

We can also use a left join to join the Employee table with itself based on team_id, and then group by employee_id to count the total number of people in each team that the employee belongs to.

SQL Code
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# Write your MySQL query statement below
SELECT e1.employee_id, COUNT(1) AS team_size
FROM
    Employee AS e1
    LEFT JOIN Employee AS e2 USING (team_id)
GROUP BY 1;