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SQL & Relational Databases Interview Questions & Answers

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

Intermediate 386 views

What are Clustered and Non-Clustered Indexes in SQL, and when should you use each?

An Index in a relational database speeds up data retrieval operations at the cost of additional write storage and update overhead. Clustered Index Determines the physical storage order of rows on disk...

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Explain SQL ACID properties and the differences between READ COMMITTED and REPEATABLE READ isolation levels.

ACID Properties: Atomicity: Transactions execute completely or abort entirely (All-or-Nothing). Consistency: Ensures database transitions only from one valid state to another, enforcing constraints. I...

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What is Database Indexing, how does a B+ Tree index work, and when should you avoid creating indexes?

A Database Index is an auxiliary data structure that improves the speed of data retrieval operations on a database table at the cost of additional storage and slower writes. Why B+ Trees are used: All...

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Explain Common Table Expressions (CTEs) and SQL Window Functions (ROW_NUMBER, RANK, DENSE_RANK) with use cases.

Common Table Expressions (CTEs): Defined using the WITH clause, CTEs create modular, temporary named result sets that make complex multi-step queries and recursive tree traversals highly readable. Win...

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Explain Database Normalization from 1NF through BCNF with real-world anomaly examples.

Database Normalization is the process of structuring a relational database to minimize data redundancy and prevent insertion, update, and deletion anomalies. 1NF (First Normal Form): Each table column...

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Explain Redis In-Memory Data Structures and Common Caching Patterns (Cache-Aside, Write-Through, Write-Behind).

Redis is an open-source, in-memory key-value data structure store utilized as a database, cache, message broker, and streaming engine. Core Data Structures: Strings: Binary-safe strings, integers, ato...

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How do INNER JOIN and LEFT JOIN differ when a customer has no orders?

An INNER JOIN returns customer-order combinations only when the join condition matches. A LEFT JOIN retains every customer from the left side and uses NULL for order columns when no match exists. Plac...

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How should SQL queries test for NULL, and why is NULL = NULL not true?

NULL represents missing or unknown information, and ordinary comparisons involving NULL evaluate to unknown rather than true or false. Use IS NULL or IS NOT NULL to test for it. WHERE keeps only rows ...

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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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