The Forth Normal Form - 4NF

In order to bring a table in the forth normal form - 4NF, we will first have to put it in third normal form 3NF, and then to remove all multi-valued dependencies which may exist among its fields.

Transforming a table to Boyce-Codd normal form (BCNF), we are sure that its structure does not contain functional dependencies . But sometimes a relationship could appear between two fields one of which has multi-valued dependencies. The table COURSES whose an instance of data is presented below is in Boyce-Codd normal form having a primary key that is composed by the combination of the tree fields lesson, instructor and textbook since all fields that identify other fields belong to the primary key. But, as you can see there are even data which all cause abnormal situations which we have mentioned previously. In our data instant example, a lesson could be taught by one or more instructors who can select their favorite textbook based on their teaching. The schema of the this table in the 3NF or BCNF will be the following COURSES {lesson, instructor, textbook} In order to remove these multi-valued dependencies we will have to break the initial table COURSES to smaller tables which will be structured in the following schema. TEACHER {instructor, lesson}     BOOKS {textbook, lesson} Now, these tables are in the 4NF.