From Relational Databases to Deductive Databases

Most of the contemporary database systems are based on the relational model of data, thus SQL. There are many well tested ways of query optimization for SQL, and many applications that use it, not mentioning the number of qualified SQL, or in general, database programmers.

SQL is designed for the relational model. Relational algebra handles data only, it does not anticipate a need for intensional knowledge nor does SQL.

However, there are some attempts to increase SQL expressive power towards this of DDB. The most significant one is an extension which allows recursive queries. It is described by SQL99, but it has not been implemented in many RDBMS. Actually, the only RDBMS that supports it, is DB2 by IBM [16].

The distance from enabling recursive queries to intensional knowledge processing is very long, and basically it is not possible to make SQL support it without some major redesign. A major redesign causes major changes and presumably incompatibilities regarding already running DBMS. These incompatibilities prevent the changes. No company can afford such a major switch from one technology to another.