The logic data language (LDL) system provides a declarative logic-based language and integrates relational database and logic programming technologies so as to support advanced data and knowledge-based applications. A comprehensive overview of the system and a description of LDL language and the compilation techniques employed to translate LDL queries into target query execution plans on the stored data are presented. The architecture and runtime environment of the system and the optimization techniques employed in order to improve the performance and assure the safety of the compiled queries are given. The experience gained so far with the system and application areas where the LDL approach appears to be particularly effective are discussed. >