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What does a LEFT JOIN combined with WHERE right_table.key IS NULL do in SQL?

easy📝 Conceptual Q11 of 15
SQL - LEFT and RIGHT JOIN
What does a LEFT JOIN combined with WHERE right_table.key IS NULL do in SQL?
AFinds rows in the right table that have no matching rows in the left table
BFinds rows in the left table that have no matching rows in the right table
CReturns all rows from both tables regardless of matches
DDeletes unmatched rows from the left table
Step-by-Step Solution
Solution:

  1. Step 1: Understand LEFT JOIN behavior

    A LEFT JOIN returns all rows from the left table and matching rows from the right table. If no match exists, right table columns are NULL.

  2. Step 2: Apply WHERE condition to filter unmatched rows

    Filtering with WHERE right_table.key IS NULL selects only those left table rows without a match in the right table.

  3. Final Answer:

    Finds rows in the left table that have no matching rows in the right table -> Option B

  4. Quick Check:

    LEFT JOIN + IS NULL = unmatched left rows [OK]
Quick Trick: LEFT JOIN + IS NULL filters unmatched left table rows [OK]
Common Mistakes:
MISTAKES
  • Confusing unmatched rows as from right table
  • Using INNER JOIN instead of LEFT JOIN
  • Not checking for NULL in right table columns

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