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SQL Mastery
Essential SQL queries, joins, aggregations, and optimization techniques for developers.
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What is the difference between INNER JOIN and LEFT JOIN?
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INNER JOIN returns only matching rows from both tables. LEFT JOIN returns all rows from the left table plus matching rows from the right (NULLs for no match).
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What is a database index and why use one?
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A data structure (usually B-tree) that speeds up lookups on a column. Trade-off: faster reads but slower writes and more storage.
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What is the difference between WHERE and HAVING?
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WHERE filters rows before grouping. HAVING filters groups after GROUP BY. Use HAVING for conditions on aggregate functions.
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What is normalization?
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Organizing data to reduce redundancy. Normal forms: 1NF (atomic values), 2NF (no partial dependencies), 3NF (no transitive dependencies).
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What is a transaction and ACID?
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A unit of work that is Atomic (all-or-nothing), Consistent (valid state), Isolated (concurrent safety), and Durable (persisted after commit).
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What is a subquery vs a CTE?
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Subquery is a nested SELECT. CTE (Common Table Expression) uses WITH clause for readability. CTEs can be recursive; subqueries cannot.
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What does EXPLAIN do?
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Shows the query execution plan — which indexes are used, join strategies, estimated row counts. Essential for query optimization.
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What is the difference between DELETE, TRUNCATE, and DROP?
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DELETE removes rows (can use WHERE, logged). TRUNCATE removes all rows (faster, minimal logging). DROP removes the entire table structure.
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