SQL - LEFT and RIGHT JOIN
Identify the error in this SQL query:
SELECT a.id, b.value FROM A a LEFT JOIN B b ON a.id = b.id WHERE b.value > 10;
Identify the error in this SQL query:
medium📝 Debug Q14 of 15
Step-by-Step Solution
Solution:
Step 1: Understand LEFT JOIN with WHERE filter
The WHERE clause filters rows after join. Filtering on b.value > 10 excludes rows where b.value is NULL.Step 2: Effect on LEFT JOIN
This filtering removes rows without matches in B, making LEFT JOIN behave like INNER JOIN.Final Answer:
The WHERE clause filters out rows where b.value is NULL, negating LEFT JOIN effect -> Option DQuick Check:
Filtering on right table in WHERE breaks LEFT JOIN [OK]
Quick Trick: Use ON for right table filters, not WHERE, to keep LEFT JOIN effect [OK]
Common Mistakes:
MISTAKES- Filtering right table columns in WHERE after LEFT JOIN
- Confusing ON and WHERE clauses
- Assuming LEFT JOIN always keeps all left rows regardless of WHERE
Master "LEFT and RIGHT JOIN" in SQL
Want More Practice?
15+ quiz questions · All difficulty levels · Free
Free Signup - Practice All QuestionsMore SQL Quizzes