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===Primary key===
{{Main|Unique key|Primary key}}
Every [[Relation (database)|relation]]/table has a primary key, this being a consequence of a relation being a [[Set (mathematics)|set]].{{Sfnp|Date|1984|p=268}} A primary key uniquely specifies a tuple within a table. While natural attributes (attributes used to describe the data being entered) are sometimes good primary keys, [[surrogate key]]s are often used instead. A surrogate key is an artificial attribute assigned to an object which uniquely identifies it (for instance, in a table of information about students at a school they might all be assigned a student ID in order to differentiate them). The surrogate key has no intrinsic (inherent) meaning, but rather is useful through its ability to uniquely identify a tuple.
Another common occurrence, especially in regard to N:M cardinality is the [[Compound key|composite key]]. A composite key is a key made up of two or more attributes within a table that (together) uniquely identify a record.<ref>{{Cite book |last1=Connolly |first1=Thomas M |title=Database systems: a practical approach to design, implementation, and management |last2=Begg |first2=Carolyn E |publisher=Pearson |year=2015 |isbn=978-1-292-06118-4 |edition=global |location=Boston Columbus Indianapolis |pages=416}}</ref>