1789. Primary Department for Each Employee

Description of Problem

Table: Employee

+---------------+---------+
| Column Name   |  Type   |
+---------------+---------+
| employee_id   | int     |
| department_id | int     |
| primary_flag  | varchar |
+---------------+---------+
(employee_id, department_id) is the primary key (combination of columns with unique values) for this table.
employee_id is the id of the employee.
department_id is the id of the department to which the employee belongs.
primary_flag is an ENUM (category) of type ('Y', 'N'). If the flag is 'Y', the department is the primary department for the employee. If the flag is 'N', the department is not the primary.

Employees can belong to multiple departments. When the employee joins other departments, they need to decide which department is their primary department. Note that when an employee belongs to only one department, their primary column is 'N'.

Write a solution to report all the employees with their primary department. For employees who belong to one department, report their only department.

Return the result table in any order.

The result format is in the following example.

Example 1:

Input: 
Employee table:
+-------------+---------------+--------------+
| employee_id | department_id | primary_flag |
+-------------+---------------+--------------+
| 1           | 1             | N            |
| 2           | 1             | Y            |
| 2           | 2             | N            |
| 3           | 3             | N            |
| 4           | 2             | N            |
| 4           | 3             | Y            |
| 4           | 4             | N            |
+-------------+---------------+--------------+
Output: 
+-------------+---------------+
| employee_id | department_id |
+-------------+---------------+
| 1           | 1             |
| 2           | 1             |
| 3           | 3             |
| 4           | 3             |
+-------------+---------------+
Explanation: 
- The Primary department for employee 1 is 1.
- The Primary department for employee 2 is 1.
- The Primary department for employee 3 is 3.
- The Primary department for employee 4 is 3.

Solution

Tags: SQL Joins

Code 1 (MySQL)

SELECT e1.employee_id, e1.department_id
FROM Employee e1
WHERE NOT EXISTS (
    SELECT 1 
    FROM Employee e 
    WHERE e.employee_id = e1.employee_id 
    AND primary_flag = 'Y'
)
AND 1 = (
    SELECT COUNT(1)
    FROM Employee e 
    WHERE e.employee_id = e1.employee_id 
    AND primary_flag = 'N'
)

UNION

SELECT e2.employee_id, e2.department_id
FROM Employee e2
WHERE e2.primary_flag = 'Y'
;

Code 2 (MySQL) - Optimised

SELECT e1.employee_id, e1.department_id
FROM Employee e1
GROUP BY e1.employee_id
HAVING COUNT(e1.department_id) = 1

UNION

SELECT e2.employee_id, e2.department_id
FROM Employee e2
WHERE e2.primary_flag = 'Y'
;