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Some years after publication of his 1970 model, Codd proposed a [[three-valued logic]] (True, False, Missing/[[Null (SQL)|NULL]]) version of it to deal with missing information, and in his ''The Relational Model for Database Management Version 2'' (1990) he went a step further with a four-valued logic (True, False, Missing but Applicable, Missing but Inapplicable) version.<ref>{{cite book| first =Christopher J. | last = Date|title=Date on Database: Writings 2000–2006|date= 2006 |publisher= Apress|isbn= 978-1-59059-746-0|pages=329–41|chapter= 18. Why Three- and Four-Valued Logic Don't Work}}</ref>
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=== Basic
[[File:Relational database terms.svg|thumb|A relation with 5 attributes (its degree) and 4 tuples (its cardinality) can be visualized as a table with 5 columns and 4 rows. However, unlike rows and columns in a table, a relation's attributes and tuples are unordered.]]
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An attribute may be unique across tuples without being a key. For example, a relation describing a company's employees may have two attributes: ID and Name. Even if no employees currently share a name, if it is possible to eventually hire a new employee with the same name as a current employee, the attribute subset {Name} is not a key. Conversely, if the subset {ID} is a key, this means not only that no employees ''currently'' share an ID, but that no employees ''will ever'' share an ID.<ref name="professionals"/>{{rp|31–33}}
==== Foreign
A ''foreign key'' is a subset of attributes ''{A}'' in a relation ''R<sub>1</sub>'' that corresponds with a key of another relation ''R<sub>2</sub>'', with the property that the [[Projection (relational algebra)|projection]] of ''R<sub>1</sub>'' on ''{A}'' is a subset of the projection of ''R<sub>2</sub>'' on ''{A}''. In other words, if a tuple in ''R<sub>1</sub>'' contains values for a foreign key, there must be a corresponding tuple in ''R<sub>2</sub>'' containing the same values for the corresponding key.<ref name="professionals"/>{{rp|34}}
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