Solution Explanation for LeetCode Problem 1303: Find the Team Size

This problem requires finding the size of each team an employee belongs to. The solution involves using SQL queries to aggregate data from the Employee table. Two approaches are presented: one using a common table expression (CTE) and another utilizing a self-join.

Approach 1: Using a CTE (Common Table Expression)

This approach first creates a CTE named T to calculate the team size for each team_id. It groups the Employee table by team_id and counts the number of employees in each team using COUNT(1). The result is a table with team_id and the corresponding team_size.

Then, it joins the original Employee table with the T CTE using team_id as the join key. This ensures that each employee's record is associated with the team_size calculated in the CTE. Finally, it selects the employee_id and team_size from the joined result.

MySQL Code:

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);

Time Complexity Analysis:

• The CTE (T) creation involves a GROUP BY operation, which has a time complexity of O(N log N) or O(N) depending on the database implementation. N is the number of rows in the Employee table.
• The JOIN operation has a time complexity of O(M log M) or O(M), where M is the size of the intermediate table generated by the CTE (which is less than or equal to N).
• Overall, the dominant factor is the GROUP BY operation, making the time complexity approximately O(N log N) or O(N) in the best case.

Approach 2: Using a Self-Join

This approach employs a LEFT JOIN operation to join the Employee table with itself. Both instances of the table (aliased as e1 and e2) are joined using team_id. This essentially creates pairs of employees within the same team.

Then, a GROUP BY clause groups the results by e1.employee_id, and COUNT(1) counts the number of employees in each group (team). This gives the team_size for each employee.

MySQL Code:

SELECT e1.employee_id, COUNT(1) AS team_size
FROM
    Employee AS e1
    LEFT JOIN Employee AS e2 USING (team_id)
GROUP BY 1;

Time Complexity Analysis:

• The LEFT JOIN operation has a similar time complexity to the JOIN in Approach 1: O(N log N) or O(N).
• The GROUP BY operation again contributes O(N log N) or O(N) time complexity.
• The overall time complexity is approximately O(N log N) or O(N), similar to Approach 1.

Comparison of Approaches:

Both approaches achieve the same result. The CTE approach might be slightly more readable as it separates the team size calculation from the final join, making the logic clearer. However, the performance difference between the two approaches is usually negligible for reasonably sized datasets. The choice depends largely on personal preference and coding style.