Некоторые реляционные операции, например, объединения, пересечения и взятия разности, требуют, чтобы отношения имели одинаковые заголовки.

In relational algebra, a projection is a unary operation written as Πa1,...,an(R){\displaystyle \Pi _{a_{1},...,a_{n}}(R)} where a1,...,an{\displaystyle a_{1},...,a_{n}} is a set of attribute names. The result of such projection is defined as the set obtained when the components of the tuple R{\displaystyle R} are restricted to the set {a1,...,an}{\displaystyle \{a_{1},...,a_{n}\}} – it discards (or excludes) the other attributes.

In set theory, a projection is one of two closely related types of functions or operations, namely: