How to achieve SQL in approximate divide and exact relation division by Relational algebra

摘要

Relational algebra is the basis of relational database theory, however, SQL is a concrete realization of the standard computer language. Particularly, the division problem in Relational algebra, in many circumstances, it's just a form of the abstract theory. By taking a further step of analyzing relational algebra, it will be devided into two parts: approximate divide and exact relation division, the paper will achieve SQL in approximate divide and exact relation division by Relational algebra.