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Data & Analytics · Advanced

Covering Index

An index that contains every column a query needs, so the query can be answered from the index alone.

Definition

Covering indexes eliminate lookups to the base table. In SQL Server, non-key columns can be added with INCLUDE, which keeps the key narrow while still covering the query.

Example

CREATE INDEX IX_Orders_Cust ON Orders (CustomerID) INCLUDE (OrderDate, Total);

Exam tip

Use INCLUDE for columns that are only selected; use key columns for columns that are filtered or sorted on.

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