Explain anti-joins and how to implement them in SQL
This question tests knowledge of finding non-matching records between tables.
Why Interviewers Ask This
Anti-joins are essential for finding missing data or exclusions. Interviewers assess if you know the standard patterns (LEFT JOIN...IS NULL or NOT EXISTS).
How to Answer This Question
Define anti-join as returning rows from one table with no match in another. Show implementation via LEFT JOIN followed by WHERE IS NULL or using NOT EXISTS. Explain the 'in A but not B' logic.
Key Points to Cover
- Finding non-matches
- LEFT JOIN pattern
- NOT EXISTS alternative
- Orphan detection
Sample Answer
An anti-join returns rows from one table that have no matching rows in another, essentially finding 'in A but not B'. It is commonly expressed as a LEFT JOIN followed by WHERE right_table.key IS NULL, or using a NOT EXIS…
Common Mistakes to Avoid
- Using INNER JOIN by mistake
- Missing NULL checks
- Confusing with semi-joins
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