What are SQL joins and what are the main types?
This question tests your ability to combine data from multiple tables. It covers INNER, LEFT, RIGHT, and FULL OUTER joins and their specific behaviors.
Why Interviewers Ask This
Joining tables is the most common task in database work. Interviewers ask this to ensure you can retrieve related data correctly. Misunderstanding join types leads to missing data or duplicate rows, which causes significant business logic errors in applications.
How to Answer This Question
Define a join as combining rows from two tables based on a matching condition. Briefly define INNER JOIN (matches only), LEFT JOIN (all left + matches), RIGHT JOIN (all right + matches), and FULL JOIN (all rows from both). Use a Venn diagram analogy if helpful, focusing on what happens when there is no match.
Key Points to Cover
- INNER JOIN returns intersection of tables
- LEFT JOIN keeps all left table rows
- FULL JOIN returns all rows from both
Sample Answer
SQL joins combine rows from two tables based on a key relationship. An INNER JOIN returns only rows that exist in both tables. A LEFT JOIN returns all rows from the left table and matching rows from the right, filling NU…
Common Mistakes to Avoid
- Confusing LEFT and RIGHT join directions
- Forgetting about NULL handling in outer joins
- Using CROSS JOIN when a specific match is needed
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