1789. Primary Department for Each Employee.sql

# Write your MySQL query statement below
SELECT 
    employee_id, department_id 
FROM 
    Employee 
WHERE 
    primary_flag = "Y"
UNION 
    SELECT 
        employee_id, department_id 
    FROM 
        Employee 
    GROUP BY
        employee_id
    HAVING 
        COUNT(employee_id) = 1;