A tuple is a single element of a relation. Formally, given a relation schema with associated domains , a tuple is an element of the Cartesian product:

That is, a tuple is an ordered assignment of values to each attribute of the schema, where each value is drawn from the corresponding domain.

Notation

For a tuple and an attribute , we write (or equivalently ) to denote the value of in . For a set of attributes , we write for the projection of onto .

Properties

• A relation is a set of tuples, so no two tuples in a relation are identical (no duplicate rows).
• Two tuples and are equal on attribute set if , meaning they agree on every attribute in . This notation is central to the definition of functional dependence.