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Asked: 2026-06-15T16:41:06+00:00

Why can’t I do something like the following? I want filters on this common

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Why can’t I do something like the following? I want filters on this “common” column to be both fast and to also be able to return it without requiring a table scan.

CREATE NONCLUSTERED INDEX Ix_Foo
ON Foo (ForeignKeyID, ObjectID)
INCLUDE (ObjectID)

When I do this, I get the error:

Cannot use duplicate column names in index. Column name ‘ObjectID’ listed more than once.

I’d like this for queries such as this where I both want to return ObjectID as well as filter by it. The subquery here is an example of what I mean:

SELECT something FROM Bar
WHERE Bar.FooID IN
  (SELECT ObjectID FROM Foo WHERE ForeignKeyID=13 AND ObjectID IN (12, 13, 14, 15))

What am I conceptually missing?

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1 Answer

  1. Editorial Team

    The reason is ObjectId is already included into the index as a key column and you’re trying to include it as a non-key one too, which is unnecessary.

    CREATE NONCLUSTERED INDEX Ix_Foo
ON Foo (ForeignKeyID, ObjectID)

    This should be enough for your purposes.

    You’d normally need to include non-key columns so that (quoting MSDN):

    • They can be data types not allowed as index key columns.

    • They are not considered by the Database Engine when calculating the number of index key columns or index key size.

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