Solution:Step 1: Understand effect of WHERE on LEFT JOINWHERE filters after join, so rows with NULL b.value are removed, losing left rows.Step 2: Identify how to fix to keep all left rowsMove condition to ON clause or use WHERE b.value > 10 OR b.value IS NULL to preserve unmatched rows.Final Answer:

[Solved] Consider this query:SELECT a.id, b.value FROM A LEFT JOIN B ON a.id = b.a_id WHERE b.value > 10;What is the problem with this query if you want to keep all rows from A? | SQL

SQL - LEFT and RIGHT JOIN

Consider this query:

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

What is the problem with this query if you want to keep all rows from A?

AThe WHERE clause filters out rows where b.value is NULL, losing some left rows.

BThe ON clause is missing a join condition.

CLEFT JOIN should be replaced with INNER JOIN for correct results.

DThe SELECT statement is missing table aliases.

Step-by-Step Solution

Solution:

Step 1: Understand effect of WHERE on LEFT JOIN
WHERE filters after join, so rows with NULL b.value are removed, losing left rows.
Step 2: Identify how to fix to keep all left rows
Move condition to ON clause or use WHERE b.value > 10 OR b.value IS NULL to preserve unmatched rows.
Final Answer:
The WHERE clause filters out rows where b.value is NULL, losing some left rows. -> Option A
Quick Check:
WHERE after LEFT JOIN can remove unmatched rows [OK]

Quick Trick: Put filters on right table in ON, not WHERE, to keep all left rows [OK]

Common Mistakes:

MISTAKES

Assuming WHERE doesn't affect LEFT JOIN results
Confusing ON and WHERE clauses
Replacing LEFT JOIN with INNER JOIN unnecessarily

Master "LEFT and RIGHT JOIN" in SQL

9 interactive learning modes - each teaches the same concept differently

Learn Why Deep Visual Try Challenge Project Recall Time

More SQL Quizzes

Consider this query:

Step 1: Understand effect of WHERE on LEFT JOIN

Step 2: Identify how to fix to keep all left rows

Final Answer:

Quick Check:

Want More Practice?