The algebraic specification of information systems (including databases) has been advanced by the introduction of category theoretic sketches and in particular by the authors' Sketch Data Model (SkDM). The SkDM led to a new treatment of view updating using universal properties already studied in category theory. We call the new treatment succinctly "universal updating". This paper outlines the theory of universal updating and studies the relationships between it and recent theoretical results of Hegner and Lechtenborger which in turn studied the classical" constant complement" approach to view updates. The main results demonstrate that constant complement updates are universal, that on the other hand there are sometimes universal updates even in the absence of constant complements, and that in the SkDM constant complement updates are reversible. We show further that there may be universal updates which are reversible even for views which have no complement. In short, the universal updates provide an attractive option including reversibility, even when constant complements are not available. The paper is predominantly theoretical studying different algebraic approaches to information system software but it also has important practical implications since it shows that universal updates have important properties in common with classical updates but they may be available even when classical approaches fail.
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