The purpose of consistency constraints (CCs) is to restrict the database states (or sequences

thereof) to those that are considered as legal. Therefore, constraints as present in the modeled

miniworld have to be expressed in a formal way, i.e., must be made known to the DBS.

In general, consistency constraints can be classified according to where they are specified

into internal and external consistency constraints.

Internal CCs are those that are known to and can be enforced by the DBS, while external

CCs have to be expressed, checked, and enforced within application programs. Internal CCs

can be subdivided into the following three types [7]:

• inherent consistency constraints: they are fixed for a given data model and therefore do not

have to be specified (e.g., absence of cycles in inheritance relationships),

• implicit consistency constraints: they can be specified in the schema using specific constructs

(keywords) of the data definition language (DDL), e.g., the identification of unique

properties,

• explicit consistency constraints: they are formulated in a separate sublanguage of the DDL,

the constraint definition language, either as predicates in a declarative language or by

means of triggers. As an example, consider the typical consistency constraint that employees

may not earn more than their manager.

Another classification of consistency constraints is based on whether only single database

states or even database state transitions can