Dependency theory is a subfield of database theory

Database theory

Database theory encapsulates a broad range of topics related to the study and research of the theoretical realm of databases and database management systems....

 which studies implication and optimization problems related to logical constraints, commonly called dependencies, on databases.
The best known class of such dependencies are functional dependencies, which form the foundation of keys

Candidate key

In the relational model of databases, a candidate key of a relation is a minimal superkey for that relation; that is, a set of attributes such that# the relation does not have two distinct tuples In the relational model of databases, a candidate key of a relation is a minimal superkey for that...

 on database relations. Another important class of dependencies are the multivalued dependencies

Multivalued dependency

In database theory, multivalued dependency is a full constraint between two sets of attributes in a relation.In contrast to the functional dependency, the multivalued dependency requires that certain tuples be present in a relation. Therefore, a multivalued dependency is a special case of...

. A key algorithm in dependency theory is the Chase

Chase (algorithm)

The Chase is a simple fixpoint algorithm testing and enforcing implication of data dependencies in database systems. It plays important roles in database theory as well as in practice....

, and much of the theory is devoted to its study.