1789-primary-department-for-each-employee

1789 https://leetcode.cn/problems/primary-department-for-each-employee/

(
    SELECT t.employee_id, t.department_id
    FROM Employee t
    INNER JOIN (SELECT employee_id, department_id FROM Employee WHERE primary_flag='Y') s
        ON t.employee_id = s.employee_id AND t.department_id = s.department_id
)
UNION
(
    SELECT t.employee_id, t.department_id
    FROM Employee t
    INNER JOIN (SELECT employee_id, COUNT(*) AS CNT FROM Employee GROUP BY employee_id) s
        ON t.employee_id = s.employee_id AND s.CNT=1
)