What is an anti-join and how do you implement it in SQL?
This question tests your knowledge of finding non-matching rows between tables. It evaluates alternative ways to express exclusion logic.
Why Interviewers Ask This
Finding differences between datasets is a common requirement. Interviewers ask this to see if you know standard patterns like LEFT JOIN ... IS NULL or NOT EXISTS. It demonstrates depth in understanding set logic beyond simple equality.
How to Answer This Question
Define an anti-join as returning rows from one table that have no matches in another. Explain the two common implementations: a LEFT JOIN followed by a WHERE clause checking for NULL on the right side, or using NOT EXISTS. Mention that NOT EXISTS is often clearer and more performant.
Key Points to Cover
- Returns non-matching rows
- Implement via LEFT JOIN + NULL check
- Alternative is NOT EXISTS
Sample Answer
An anti-join returns rows from the left table that do not have a corresponding match in the right table. The most common implementation is a LEFT JOIN combined with a WHERE clause checking if the right-side primary key i…
Common Mistakes to Avoid
- Using LEFT JOIN without the NULL check
- Confusing anti-join with inner join
- Not considering NULL handling in comparisons
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