SQL - INNER JOIN
Identify the error in this self join query:
SELECT e1.name, e2.name FROM employees e1 JOIN employees e2 ON e1.id = e2.id;
Identify the error in this self join query:
medium📝 Debug Q14 of 15
Step-by-Step Solution
Solution:
Step 1: Analyze the join condition
The query joins employees to itself one1.id = e2.id, which matches each row to itself only, not to related rows.Step 2: Understand the effect of the condition
This join condition does not find relationships like manager or pairs; it just duplicates rows, which is likely incorrect for self join use.Final Answer:
The join condition compares the same column, causing incorrect results -> Option DQuick Check:
Self join needs meaningful join condition, not same column equals [OK]
Quick Trick: Join condition must relate different columns for meaningful self join [OK]
Common Mistakes:
MISTAKES- Joining on identical columns returns only same rows
- Forgetting to use aliases
- Assuming JOIN requires WHERE clause
Master "INNER JOIN" in SQL
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