# Primary Keys - Clustered and Non-Clustered Indexes
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In the TiDB database, regarding the tables' unique index on the primary key, they can be classified into two categories: clustered and non-clustered indexes.

In TiDB, every table is guaranteed to have at least one unique index, which is a crucial feature provided by its storage engine, TiKV. You will learn more about TiKV as you progress through this course.