[Solved] Identify the error in this self join query:SELECT e1.name, e2.name FROM employees e1 JOIN employees e2 ON e1.manager_id = e2.id WHERE e1.id = e2.id; | PostgreSQL

PostgreSQL - Joins in PostgreSQL

Identify the error in this self join query:

SELECT e1.name, e2.name FROM employees e1 JOIN employees e2 ON e1.manager_id = e2.id WHERE e1.id = e2.id;

AIncorrect JOIN keyword used

BMissing table alias for employees

CThe WHERE clause contradicts the ON condition causing no rows

DNo error, query is valid

Step-by-Step Solution

Solution:

Step 1: Analyze ON and WHERE conditions
ON joins where e1.manager_id = e2.id, but WHERE requires e1.id = e2.id.
Step 2: Understand contradiction
e1.id cannot equal e2.id and simultaneously e1.manager_id = e2.id unless manager_id = id, which is rare; likely no rows.
Final Answer:
The WHERE clause contradicts the ON condition causing no rows -> Option C
Quick Check:
Contradictory conditions = No rows [OK]

Quick Trick: Check ON and WHERE conditions for conflicts [OK]

Common Mistakes:

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Identify the error in this self join query: