What are SQL joins and how do INNER, LEFT, RIGHT, and FULL differ?
This is a core question evaluating mastery of combining data from multiple tables effectively.
Why Interviewers Ask This
Joins are the most common operation in relational databases. Interviewers assess your ability to retrieve related data accurately and understand the nuances of different join types to avoid data loss or duplication.
How to Answer This Question
Define joins as combining rows based on matching conditions. Briefly explain each type: INNER for matches only, LEFT for all left plus matches, RIGHT for all right plus matches, and FULL for all rows from both. Use Venn diagram concepts if helpful.
Key Points to Cover
- Combining tables via conditions
- INNER vs OUTER distinctions
- Handling unmatched rows
- NULL behavior in outer joins
Sample Answer
SQL joins combine rows from two tables based on a matching condition to answer questions spanning both tables. An INNER JOIN returns only matches existing in both tables. A LEFT JOIN returns all rows from the left table…
Common Mistakes to Avoid
- Confusing LEFT and RIGHT
- Missing NULL implications
- Not distinguishing FULL JOIN
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