SEMANTIC CONSTRAINTS EXPRESSED BY NETWORK MODEL.

In this paper semantic constraints expressed by network database model are summarized. In relational model functional dependencies and a join dependency can be expressed by a set of relations. Network model is different from relational model by the following reasons; (1) The basic structure is a DBTG set not a relation, (2) ordered set can be expressed directly, (3) to improve the efficiency of query processing, redundancy is permitted, (4) multiple paths between two attributes are permitted. Semantic constraints for a set of DBTG set types correspond to multivalued and join dependencies. Correspondence between Bachman diagrams and such dependencies are given. Problems of Lien's definitions of dependencies are pointed out and new definitions are given. Multiple paths which are used to improve efficiency of query processing are also summarized.