ture and simplifies the implementation of the system, it can sometimes confuse the users since in reality there is no absolute uniformity.The design of the relational database system and the mechanisms that they use have been mathematically founded. Most of them are the result of long research periods that lead to the successful solving of the most important problems that occurred in these systems. The object-oriented database systems, since they are fairly new, do not have a very sound theoretical solution for many of the issues that arise from their implementations. Here are some of the problems introduced by the new approach:•The object-oriented model contains some concepts whose semantics are still under discussion. There is no standard data model and consequently there is no standard methodology for designing an object-oriented scheme. For the relation systems on the contrary, the ER diagram is totally acceptable.•The query language of the relational systems was base on the mathematical theory of the relational algebra and the relational calculus. There is not something similar for the OODBMS. A lot of effort has been done for the definition of an object-oriented algebra since it is clear that the relational algebra is inadequate for the support of the object-oriented model.•The traditional indexing and locking techniques used should be extended in order to be used for object-oriented databases. The composite objects cause a lot of trouble and is still an open research issue.•The complexity of the hierarchies of classes created can be so big that the schemas can be handled with difficulty.The object-oriented systems are very much successful in areas where their predecessors failed:•The design of the schema can be done in a very direct way since the object-oriented model is very close to the real world model. On the contrary, the relational design which is based on canonical forms of the relati...
