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From The Free On-line Dictionary of Computing (05 January 2017) [foldoc]:
union
1. An operation on two sets which returns the set of
all elements that are a member of either or both of the 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. A type whose values may be of one of a number
of other types, the 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. 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)