This effort is one of the few that have implemented DDDBs.
The development of model-theoretic, fixpoint, and proof procedures placed the semantics of DDDBs on a firm foundation.
Fernandez and Minker (1992) develop the theory of DDDBs using the concept of model trees.
Four alternative semantics were developed for nonstratifiable normal DDDBs at approximately the same time: (1) Ross (1989), the strong WFS; (2) Baral, Lobo, and Minker (1990), the generalized disjunctive WFS (GDWFS); and (3, 4) two semantics by Przymusinski, an extension of the stable model semantics (Przymusinski 1990) for normal disjunctive databases and the stationary semantics (Przymusinski 1990).
Efforts have been developed both for DDBs and DDDBs by Kautz and Selman (1992) and Selman and Kautz (1996), who developed lower and upper bounds for Horn (datalog) databases and compilation methods; Cadoli (1993), who developed computational and semantic approximations; and del Val (1995), who developed techniques for approximating and compiling databases.
Hence, DDDBs can be used to compute answers to queries in such theories.
Another area to which DDDBs have contributed is the null-value problem.
First is the incorporation of techniques developed in DDDBs into relational technology.