This volume presents the proceedings of the 1991 Symposium on Mathematical Fundamentals of Database and Knowledge Base Systems, held in Rostock, FRG, May 6-9, 1991. MFDBS 91 covers new developments in theoretical aspects of database and knowledge base systems and the design of databases and knowledge bases.

Complex object multi-level fixpoint queries.- Classifying object-oriented query results in a class/type lattice.- A relational algebra for complex objects based on partial information.- Specifying operations for nested relations by rules and partial orders.- On the relationship between well-founded and stable partial models.- Monadic databases with equality.- Actor-oriented specification of deontic integrity constraints.- Solving ambiguities in updating deductive databases.- On the mean execution time of recursive definitions on relational databases.- Updating hierarchical databases by complex forms.- A direct algorithm for computing the transitive closure of a two-dimensionally structured file.- Multi-granularity locking for nested transaction systems.- The Nested Interpolation Based Grid File.- Complexity of recursive production rules execution.- On the monotonicity of (LDL) logic programs with set.- Semantic improvement of deductive databases.- On the representation of dependencies by propositional logic.- Pairwise-definable subdirect decompositions of general database schemata.- Functional dependencies in hierarchical structures of data.- Objects in relational database schemes with functional, inclusion and exclusion dependencies.- Abstract data type semantics for many-sorted object query algebras.- Temporal specifications of object behavior.- Consistency and non-determinism in a database programming language.- A fact-oriented approach to schema transformation.- A database needs two kinds of negation.- Rationale of the O2 system.- The background of the DASDBS & COSMOS projects.- The rationale behind DBPL.