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.
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