On Constraints and Queries in UML

The UML has recently been extended by an Object Constraint Language (OCL). This formal language can be used for specifying constraints on a model in order to restrict possible system states. We present some examples for illustrating main concepts of OCL. Problems with the current definition of OCL resulting from imprecise or ambiguous definitions are investigated. A comparison of OCL with a language for specification of queries and integrity constraints in an Extended Entity-Relationship model shows similarities between both approaches. This comparison could lead to a better understanding of OCL.