1789. Primary Department for Each Employee.sql

/*
Question 1789. Primary Department for Each Employee
Link: https://leetcode.com/problems/primary-department-for-each-employee/description/?envType=study-plan-v2&envId=top-sql-50

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.
*/

SELECT
    e.employee_id,
    (CASE 
        WHEN COUNT(e.employee_id) = 1
            THEN (
                SELECT e1.department_id
                FROM Employee AS e1
                WHERE e1.employee_id = e.employee_id
            )
        ELSE (
            SELECT e2.department_id
            FROM Employee AS e2
            WHERE e2.employee_id = e.employee_id AND e2.primary_flag = 'Y'
        )
    END) AS department_id
FROM Employee AS e
GROUP BY e.employee_id