Object-database systems have developed along two distinct paths:
Object-oriented database systems: Object-oriented database
proposed as an alternative to relational systems and are aimed at application domains where complex objects play a central role. The approach is heavily in-�?uenced by object-oriented programming languages and can be understood as an attempt to add DBMS functionality to a programming language environment.
Object-relational database systems: Object-relational database systems can be thought of as an attempt to extend relational database systems with the functionality necessary to support a broader class of applications and, in many ways, provide a bridge between the relational and object-oriented paradigms.
The SQL:1999 standard is based on the ORDBMS model, rather than the OODBMS model.We have concentrated on developing the fundamental con-cepts, rather than on presenting SQL:1999; some of the features that we discuss are not included in SQL:1999. We have tried to be consistent with SQL:1999 for notation, although we have occasionally diverged slightly for clarity.