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SQL JOIN Interview Questions (2026)

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SQL JOIN Interview Questions

SQL JOIN Interview Questions

SQL JOIN questions test whether you can combine data across tables while preserving the right grain and relationships. Expect INNER, LEFT, RIGHT, CROSS, and FULL OUTER JOINs, plus multi-table joins and JOIN + aggregation patterns.

169Questions

43Companies

Easy 0 • Medium 169 • Hard 0Difficulty mix

Common SQL JOIN interview patterns

INNER vs LEFT JOIN logic
Anti-joins (find missing records)
Self-joins for comparisons within a table
JOIN + GROUP BY for aggregated results
Conditional joins with additional predicates
Multi-key joins across composite keys
Deduplication using JOINs

SQL JOIN interview questions

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Write SQL for social feed metrics and ties

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Calculate Weekly Event Sums from Daily Counts

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Write left-join queries with tricky filters

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Write SQL/Python for ACH fraud analytics

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Aggregate Netflix metrics in SQL

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Compute join counts and window ranks

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Find top-spend categories per customer with ranking

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Calculate posts per DAU by country today

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Compare SQL counts, windows, and NULL semantics

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Calculate Daily Harmful Content View Percentage

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Calculate Distinct High-View Posts and Spam View-Prevalence

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Label new vs old users over time in SQL

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Compute feed ad frequency and retention in SQL

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Calculate Average Session Duration and Performance Metrics

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Calculate Ad Performance with Click-Through and Conversion Rates

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Common mistakes with SQL JOINs

JOIN explosion from many-to-many relationships
Filtering in WHERE instead of ON for LEFT JOINs
Incorrect NULL handling on optional joins
COUNT(*) vs COUNT(column) confusion
Duplicate rows after joins

How JOIN questions are evaluated in interviews

Correctness beats performance; the right result set matters most.

Explain your join keys, assumptions, and how NULLs are treated.

Reasoning and clarity matter more than memorized syntax.

Related SQL concepts

SQL window function interview questions
SQL aggregation interview questions
SQL CTE and subquery interview questions
SQL NULL handling interview questions

SQL JOIN Interview FAQs

What is the difference between INNER JOIN and LEFT JOIN in interviews?

INNER JOIN returns only matching rows from both tables, while LEFT JOIN keeps all rows from the left table and fills unmatched right rows with NULLs. Interviewers often test whether you choose the correct join for required vs optional relationships.

When should I use a subquery instead of a JOIN?

Use a subquery when you need to filter or aggregate before joining, or when a correlated condition is clearer. JOINs are better for combining row-level data, while subqueries can simplify multi-step logic.