Question

Why can't a sparse column be eligible for Primary key?

Answer

As in MSDN -

A sparse column cannot be part of a clustered index or a unique primary key index. However, both persisted and nonpersisted computed columns that are defined on sparse columns can be part of a clustered key. Sparse columns require more storage space for nonnull values than the space required for identical data that is not marked SPARSE A sparse column must be nullable and cannot have the ROWGUIDCOL or IDENTITY properties.

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