Explain Common Table Expressions (CTEs) and SQL Window Functions (ROW_NUMBER, RANK, DENSE_RANK) with use cases.
Quick Revision Summary
CTEs simplify query structure. Window functions calculate aggregations across partitions while preserving individual rows; DENSE_RANK handles ties without rank gaps.
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Model Answer & Deep-Dive Explanation
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.
Window Functions: Perform calculations across a set of table rows related to the current row without collapsing the rows into a single summary output (unlike GROUP BY).
- ROW_NUMBER(): Assigns unique, strictly sequential integers (1, 2, 3, 4) regardless of ties.
- RANK(): Assigns the same rank to identical values, then skips ranks for subsequent rows (e.g. 1, 2, 2, 4).
- DENSE_RANK(): Assigns the same rank to identical values without skipping ranks (e.g. 1, 2, 2, 3).
Interviewer Evaluation Points & Scoring Rubric
- 1 1. Clarifies CTE benefits over subqueries. 2. Distinguishes ROW_NUMBER, RANK, and DENSE_RANK tie handling. 3. Shows PARTITION BY / ORDER BY syntax knowledge.
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