What is the difference between a clustered and a non-clustered index?

Short answer: Clustered Index: The data in the table is physically sorted based on the clustered index.

Explain a bit more

Each table can have only one clustered index (usually the primary key). Fast for queries that involve range-based retrieval (e.g., BETWEEN or ORDER BY). Non-Clustered Index: The data in the table is not physically sorted. The non-clustered index is stored separately from the actual data. A table can have multiple non-clustered indexes. Faster for point lookups (single value queries) but less efficient for range-based searches.

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1. Define — one clear sentence (the short answer above).
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3. Trade-off — when you would not use it.