SQL - LEFT and RIGHT JOIN
A query uses LEFT JOIN but returns fewer rows than the left table. What is a likely cause?
A query uses LEFT JOIN but returns fewer rows than the left table. What is a likely cause?
medium📝 Debug Q7 of 15
Step-by-Step Solution
Solution:
Step 1: Recall LEFT JOIN behavior
LEFT JOIN returns all left rows unless filtered out later.Step 2: Identify filtering impact
WHERE conditions on right table columns exclude rows with NULLs, reducing result rows.Final Answer:
Filtering in WHERE clause on right table columns -> Option DQuick Check:
WHERE filters after join can reduce LEFT JOIN rows [OK]
Quick Trick: WHERE on right table columns filters out unmatched rows in LEFT JOIN [OK]
Common Mistakes:
MISTAKES- Blaming LEFT JOIN syntax for fewer rows
- Ignoring WHERE clause filtering
- Assuming duplicates reduce rows
Master "LEFT and RIGHT JOIN" in SQL
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