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The clustered index key columns are used by all nonclustered indexes internally as the means to locate rows in the table. So if you define a clustered index on a column x, and nonclustered indexes—one on column a, one on b, and one on c—your nonclustered indexes are internally created on column (a, x), (b, x), and (c, x), respectively.


Cover of Training Kit (Exam 70-461): Querying Microsoft® SQL Server® 2012


Clustered indices are combined with nonclustered indices when nonclustered indices are generated.