components. The multi-valued attributes are represented by separate

components. The multi-valued attributes are represented by separate relations. A special value called Null is used to represent unknown or not applicable values of attributes in a tuple. It is also possible to devise different types of code values for different types of null value situation.

A relational database schema is a set of relation schemas and a set of integrity constraints. A relational database state is a set of relation states such that every relational database state satisfies the integrity constraints specified on relational database schema. In this context the following points merit a special consideration : (a) A particular attribute, which stands for the same real word concept, might appear in more than one relation with same or different name. For example, in vouchers relation, the account Number is represented as debit and credit whereas in accounts relation, it is represented as Code (figure .

). EmpId appearing in Employees relation is represented in Vouchers as Auth.By and Prep.By. (b) The particular real world concept appearing more than once in a relation must be represented by different names. For example, in employees relation, employee is represented as subordinate, by using EmpId and as superior by using SuperId .