[LeetCode][MySQL] 1965. Employees With Missing Information

Easy 🔗1965. Employees With Missing Information

📝문제 요약

Table: Employees

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| employee_id | int     |
| name        | varchar |
+-------------+---------+
employee_id is the column with unique values for this table.
Each row of this table indicates the name of the employee whose ID is employee_id.

Table: Salaries

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| employee_id | int     |
| salary      | int     |
+-------------+---------+
employee_id is the column with unique values for this table.
Each row of this table indicates the salary of the employee whose ID is employee_id.

Write a solution to report the IDs of all the employees with missing information. The information of an employee is missing if:

• The employee’s name is missing, or
• The employee’s salary is missing.

Return the result table ordered by employee_id in ascending order.

The result format is in the following example.

Example 1:

Input:
Employees table:
+-------------+----------+
| employee_id | name     |
+-------------+----------+
| 2           | Crew     |
| 4           | Haven    |
| 5           | Kristian |
+-------------+----------+
Salaries table:
+-------------+--------+
| employee_id | salary |
+-------------+--------+
| 5           | 76071  |
| 1           | 22517  |
| 4           | 63539  |
+-------------+--------+
Output:
+-------------+
| employee_id |
+-------------+
| 1           |
| 2           |
+-------------+
Explanation:
Employees 1, 2, 4, and 5 are working at this company.
The name of employee 1 is missing.
The salary of employee 2 is missing.

✏️문제 풀이

첫 번째 쿼리

SELECT e.employee_id
FROM Employees e LEFT JOIN Salaries s ON e.employee_id = s.employee_id
WHERE s.employee_id IS NULL

• Employees 테이블을 기준으로 LEFT JOIN을 수행
• 즉, 모든 직원 정보(Employees)를 다 가져오고, Salaries에 해당 직원의 급여가 있으면 붙여줌
• Salaries에 정보가 없는 직원은 s.employee_id가 NULL이 됨

UNION

• 두 쿼리 결과를 합침(중복 자동 제거)

두 번째 쿼리

SELECT s.employee_id
FROM Salaries s LEFT JOIN Employees e ON s.employee_id = e.employee_id
WHERE e.employee_id IS NULL

• Salaries 테이블을 기준으로 LEFT JOIN
• 급여 정보는 있는데 직원 정보가 없는 경우(e.employee_id IS NULL)를 찾음

ORDER BY

ORDER BY employee_id

• employee_id 기준으로 오름차순 정렬

💯제출 코드

SELECT e.employee_id
FROM Employees e LEFT JOIN Salaries s ON e.employee_id = s.employee_id
WHERE s.employee_id IS NULL

UNION

SELECT s.employee_id
FROM Salaries s LEFT JOIN Employees e ON s.employee_id = e.employee_id
WHERE e.employee_id IS NULL
ORDER BY employee_id