1789. Primary Department for Each Employee

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.


SELECT employee_id ,department_id 
FROM Employee 
GROUP BY employee_id 
HAVING COUNT(department_id) = 1
UNION 
SELECT employee_id ,department_id 
FROM Employee 
GROUP BY employee_id ,primary_flag
HAVING primary_flag = 'Y'

解說：使用UNION把答案和起來