*data set*(also known as a

*store*or

*database*) is a multiset

*R*: a collection (possibly with duplicates) of tuples of the form (

*x*

_{1},...,

*x*

_{n}) taken from the set product

*X*

_{1}× ... ×

*X*

_{n}. Typically, some distinguished set (e.g., the left-most in the set product) will be a set of

*keys*, so that every tuple contains a key. Whether a set is a key or not often depends on the particular paradigm and context, however.