How can you modify a self join query to find employees who do not have a manager (top-level employees)?

hard📝 Application Q9 of 15

PostgreSQL - Joins in PostgreSQL

AUse LEFT JOIN and filter WHERE manager_id IS NULL

BUse INNER JOIN and filter WHERE manager_id IS NOT NULL

CUse RIGHT JOIN and filter WHERE manager_id IS NULL

DUse FULL JOIN and filter WHERE manager_id IS NOT NULL

Step-by-Step Solution

Solution:

Step 1: Use LEFT JOIN to keep all employees
LEFT JOIN keeps all rows from the left table even if no match in right table.
Step 2: Filter employees with no manager
Employees with NULL manager_id have no manager, so filter WHERE manager_id IS NULL.
Final Answer:
Use LEFT JOIN and filter WHERE manager_id IS NULL -> Option A
Quick Check:
Find top-level employees = LEFT JOIN + NULL filter [OK]

Quick Trick: LEFT JOIN + WHERE manager_id IS NULL finds top employees [OK]

Common Mistakes:

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