Projection

Definition

Given a relation instance R over set of attributes U, and given a subset X of U, the projection of R on X – denoted by π _X (R) – is defined as a relation over set of attributes X whose tuples are the restriction of tuples of R to attributes X. That is t ∈ π _X (R) if and only if t = t′(X) for some tuple t′ of R (here t′(X) denotes the restriction of t′ to attributes X).

Key Points

The projection is one of the basic operators of the relational algebra. It operates by “restricting” the input relation to some of its columns.

The arity of the output relation is bounded by the arity of the input relation. Moreover the number of tuples in π _X (R) is bounded by the number of tuples in R. In particular, the size of π _X (R) can be strictly smaller than the size of R since different tuples of R may have the same values on attributes X.

As an example, consider a relation Goods over attributes (code, price, quantity), containing tuples {(001, 5.00, 10), (002, 5.00, 10), (003, 25.00,...