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SQL & Relational Databases Interview Questions & Answers

Joins, indexing, ACID transactions, normalization, query optimization, and triggers.

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Write a query for the second-highest distinct non-NULL salary. What happens if it does not exist?

Rank distinct salary values in descending order with DENSE_RANK and select rank 2. Filtering NULL before ranking avoids dialect-dependent placement of NULL salaries. DISTINCT on the final selected sal...

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A LEFT JOIN report counts one order for customers who have none. What caused it?

COUNT() counts the NULL-extended customer row produced by the LEFT JOIN. Count a non-nullable order identifier instead to count actual matching orders. Group by a stable customer identifier, and inclu...

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How would you choose a composite index for a filtered, ordered query?

Start with the actual predicates, sort order, selected columns, and data distribution. For a query filtering customer_id by equality and ordering by created_at and id, an index beginning with customer...

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Two transactions both see one seat remaining and both sell it. How can the database prevent overselling?

Make the availability check and decrement part of an atomic conditional update, such as updating only when remaining is greater than zero, and confirm that exactly one row was affected. Create the res...

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