Given the employees(id, name, manager_id) table, which query correctly lists all employees who manage at least one other employee?
hard📝 Application Q8 of 15
PostgreSQL - Joins in PostgreSQL
Given the employees(id, name, manager_id) table, which query correctly lists all employees who manage at least one other employee?
ASELECT e.name FROM employees e WHERE e.manager_id IS NULL;
BSELECT DISTINCT m.name FROM employees m JOIN employees e ON m.id = e.manager_id;
CSELECT e.name FROM employees e JOIN employees m ON e.id = m.manager_id WHERE e.id = m.id;
DSELECT name FROM employees WHERE manager_id = id;
Step-by-Step Solution
Solution:
Step 1: Understand the requirement
We want employees who appear as managers (id matches manager_id of others).
Step 2: Analyze options
SELECT DISTINCT m.name FROM employees m JOIN employees e ON m.id = e.manager_id; joins employees as managers (m) to employees (e) where m.id = e.manager_id, listing managers.
Step 3: Eliminate incorrect options
B lists employees without managers, C has incorrect join condition, D is invalid syntax.
Final Answer:
SELECT DISTINCT m.name FROM employees m JOIN employees e ON m.id = e.manager_id; -> Option B
Quick Check:
Join on manager_id to find managers [OK]
Quick Trick:Join employees on manager_id to find managers [OK]
Common Mistakes:
Selecting employees without managers instead of managers
Using incorrect join conditions
Not using DISTINCT to avoid duplicates
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