[Solved] You wrote this query:SELECT * FROM A FULL OUTER JOIN B ON A.ID = B.ID WHERE A.Name = 'John';But it returns fewer rows than expected. What is the likely issue? | SQL

SQL - Advanced Joins

You wrote this query:

SELECT * FROM A FULL OUTER JOIN B ON A.ID = B.ID WHERE A.Name = 'John';

But it returns fewer rows than expected. What is the likely issue?

AThe WHERE clause filters out rows where A.Name is NULL, removing unmatched rows from B.

BFULL OUTER JOIN does not support WHERE clauses.

CThe ON condition is incorrect and causes no matches.

DThe query syntax is invalid for FULL OUTER JOIN.

Step-by-Step Solution

Solution:

Step 1: Understand WHERE clause effect after FULL OUTER JOIN
WHERE filters rows after the join, so rows with NULL in A.Name (unmatched from B) are excluded.
Step 2: Identify impact on result
This removes unmatched rows from B, reducing rows returned.
Final Answer:
The WHERE clause filters out rows where A.Name is NULL, removing unmatched rows from B. -> Option A
Quick Check:
WHERE filters after join, can exclude NULLs = A [OK]

Quick Trick: Use WHERE carefully with FULL OUTER JOIN to avoid losing unmatched rows [OK]

Common Mistakes:

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