[Solved] Consider this SQL query:SELECT a.id, b.value FROM A a LEFT JOIN B b ON a.id = b.a_id WHERE b.value > 10;Why might this query return fewer rows than table A has? | SQL

SQL - LEFT and RIGHT JOIN

Consider this SQL query:

SELECT a.id, b.value FROM A a LEFT JOIN B b ON a.id = b.a_id WHERE b.value > 10;

Why might this query return fewer rows than table A has?

ABecause the query syntax is invalid and causes an error.

BBecause LEFT JOIN only returns rows with matching b.value > 10.

CBecause the WHERE clause filters out rows where b.value is NULL, removing unmatched rows.

DBecause the ON condition is incorrect and causes no matches.

Step-by-Step Solution

Solution:

Step 1: Understand LEFT JOIN with WHERE filter
LEFT JOIN keeps all rows from A, but WHERE filters after join.
Step 2: Effect of WHERE on NULLs from unmatched rows
Rows with no match have b.value as NULL, and WHERE b.value > 10 excludes NULLs, removing those rows.
Final Answer:
Because the WHERE clause filters out rows where b.value is NULL, removing unmatched rows. -> Option C
Quick Check:
WHERE filters NULLs after LEFT JOIN, reducing rows [OK]

Quick Trick: WHERE on right table column after LEFT JOIN filters out NULLs [OK]

Common Mistakes:

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