26 Adding Valid Time to SQL/Temporal Richard T. Snodgrass, Michael H. Böhlen, Christian S. Jensen, and Andreas Steiner This change proposal specifies the addition of tables with valid-time support into SQL/Temporal, and explains how to use these facilities to migrate smoothly from a conventional relational system to a temporal system. Initially, important requirements to a temporal system that may facilitate such a transition are moti- vated and discussed. The proposal then describes the language additions neces- sary to add valid-time support to SQL3 while fulfilling these requirements. The constructs of the language are divided into four levels, with each level adding increased temporal functionality to its predecessor. The proposal formally de- fines the semantics of the query language by providing a denotational semantics mapping to well-defined algebraic expressions. Several alternatives for imple- menting the language constructs are listed. A prototype system implementing these constructs on top of a conventional DBMS is publicly available. 665