Projection

Projection, projector, or projective may refer to:

Projection (relational algebra)

In relational algebra, a projection is a unary operation written as where is a set of attribute names. The result of such projection is defined as the set obtained when the components of the tuple are restricted to the set – it discards (or excludes) the other attributes.

In practical terms, it can be roughly thought of as picking a sub-set of all available columns. For example, if the attributes are (name, age), then projection of the relation {(Alice, 5), (Bob, 8)} onto attribute list (age) yields {5,8} – we have discarded the names, and only know what ages are present.

In addition, projection can be used to modify an attribute's value: if relation R has attributes a, b, and c, and b is a number, then will return a relation nearly the same as R, but with all values for 'b' shrunk by half.

Related concepts

The closely related concept in set theory (see: projection (set theory)) differs from that of relational algebra in that, in set theory, one projects onto ordered components, not onto attributes. For instance, projecting onto the second component yields 7.

Glossary of order theory

This is a glossary of some terms used in various branches of mathematics that are related to the fields of order, lattice, and domain theory. Note that there is a structured list of order topics available as well. Other helpful resources might be the following overview articles:

In the following, partial orders will usually just be denoted by their carrier sets. As long as the intended meaning is clear from the context, ≤ will suffice to denote the corresponding relational symbol, even without prior introduction. Furthermore, < will denote the strict order induced by ≤.

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