union

1. set theory An operation on two sets which returns the

set of all elements that are a member of either or both of the

argument sets; normally written as an infix upper-case U

symbol. The operator generalises to zero or more sets by

taking the union of the current partial result (initially the

empty set) with the next argument set, in any order.

For example, (a, b, c) U (c, d, e) = (a, b, c, d, e)

2. programming A type whose values may be of one of a

number of other types, thet current type depending on

conditions that are only known at run-time. A variable of

union type must be allocated sufficient storage space to hold

the largest component type. Some unions include extra

information to say which type of value the union currently has

(a "tagged union"), others rely on the program to keep track

of this independently.

A union contrasts with a structure or record which stores

values of all component types at once.

3. database An SQL operator that concatenates two result

sets, that must have the same number and types of columns.

The operator may be followed by the word "ALL" to indicate

that results that appear in both sets should appear twice in

the output.

(2002-02-26)