To identify an employee's primary department, you must handle two distinct scenarios based on the input data:

• If an employee is assigned to multiple departments, retrieve the record where primary_flag is set to 'Y'.
• If an employee is assigned to only one department, retrieve that record regardless of the primary_flag value.

Approach 1: Using the UNION Operator

The UNION operator combines result sets from multiple queries. By splitting the logic into two separate sub-queries, we can cleanly extract the required data points:

SELECT employee_id, department_id 
FROM Employees 
GROUP BY employee_id 
HAVING COUNT(employee_id) = 1

UNION

SELECT employee_id, department_id 
FROM Employees 
WHERE primary_flag = 'Y';

This method ensures that single-department employees are caught by the first query, while primary departments for multi-department employees are caught by the second, all while automatically removing duplicates.

Approach 2: Using Window Functions

Window functions allow you to perform calculations across a set of table rows that are related to the current row without collapsing them into a single output row. By using COUNT() OVER(), we can calculate the total number of departments per employee while keeping the individual row details accessible.

SELECT employee_id, department_id
FROM (
   SELECT 
       employee_id, 
       department_id, 
       primary_flag, 
       COUNT(*) OVER (PARTITION BY employee_id) AS total_depts
   FROM Employees
) AS subquery
WHERE total_depts = 1 OR primary_flag = 'Y';

In this solution:

• PARTITION BY employee_id ensures the count is specific to each worker.
• The outer query filters the result set to include only those rows that satisfy your business logic, efficiently avoiding the need for complex joins or temporary tables.