The EXPAND and COLLAPSE Operators
Temporal databases involve not just intervals as such but, more generally, sets of such intervals. For example, projecting a relation on an interval attribute yields a set of tuples whose component values taken together constitute such a set of intervals. In order to reason about (and systematically deal with) such sets of intervals, it’s convenient to introduce two canonical forms, called expanded form and collapsed form, respectively. This chapter explains these canonical forms in detail and introduces the associated operators EXPAND and COLLAPSE. Each of these operators takes a set of intervals all of the same interval type as input and returns the corresponding canonical form of that set as output. ...
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