Are SQL Server, DB2, and Oraclereally relational?

Today, we have a huge quantity of information in the world. The development of information systems makes it possible to manage our needs. Database Management Systems (DBMS) have played a great role in the success of information systems. The most influential theory in DBMS is the Relational Database Model (RDM), which was established by E. J. Codd in the early 1970s. In this thesis, three database products, SQL Server, DB2, and Oracle are compared, which is unique because in earlier studies they have only been evaluated one database at a time. They are called Relational Database Management Systems (RDBMS), based on RDM. Their total market share is more than 80 % of the whole RDBMS. Codd points out important requirements for RDBMS, known as Codd’s twelve rules. The purpose of this study is to evaluate if and how closely each database vendor has followed Codd’s rules when developing their products. In summary, eight of the rules are put into practice by all of the vendors. The most difficult rules to implement are: 1. modification of the view table and 2. handling of null values. Our conclusions are that all three databases qualify as relational, and they have equal difficulties with the update view function and handling of null values.