A formal framework is introduced for studying the semantics and expressiven
ess of active databases. The power of various abstract trigger languages is
characterized and related to several major active database prototypes: ARD
L, HiPAC, Postgres, Starburst, and Sybase. This allows us to formally compa
re the expressiveness of the prototypes. The results provide insight into t
he programming paradigm of active databases, the interplay of various featu
res, and their impact on expressiveness and complexity. (C) 1998 Academic P
ress.