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)
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