Normal forms are used for the process of normalization of data and therefore for the database design. In theory, there are at least five different normal forms, of which the first three are the most important for practical use. The third normal form for a table can be achieved by testing the first and second normal forms at the intermediate states, and as such, the goal of good database design can usually be fulfilled if all tables of a database are in the third normal form.
The multivalued dependency is used to test the fourth normal form of a table. Therefore, this kind of dependency will not be used further in this book.
First Normal Form
First normal form (1NF) means that a table has no multivalued attributes or composite attributes. (A composite attribute contains other attributes and can therefore be divided into smaller parts.) All relational tables are by definition in 1NF, because the value of any column in a row must be atomic—that is, single valued.
Table 1 demonstrates 1NF using part of the works_on table from the sample database. The rows of the works_on table could be grouped together, using the employee number. The resulting Table 2 is not in 1NF because the column project_no contains a set of values (p1, p3).
Table 1 Part of the works_on Table
Table 2 This “Table” Is Not in 1NF
Second Normal Form
A table is in second normal form (2NF) if it is in 1NF and there is no nonkey column dependent on a partial primary key of that table. This means if (A,B) is a combination of two table columns building the key, then there is no column of the table depending either on only A or only B.
For example, Table 3 shows the works_on1 table, which is identical to the works_on table except for the additional column, dept_no. The primary key of this table is the combination of columns emp_no and project_no. The column dept_no is dependent on the partial key emp_no (and is independent of project_no), so this table is not in 2NF. (The original table, works_on, is in 2NF.)
Table 3 The works_on1 Table
Every table with a one-column primary key is always in 2NF.
Third Normal Form
A table is in third normal form (3NF) if it is in 2NF and there are no functional dependencies between nonkey columns. For example, the employee1 table (see Table 4), which is identical to the employee table except for the additional column, dept_name, is not in 3NF, because for every known value of the column dept_no the corresponding value of the column dept_name can be uniquely determined. (The original table, employee, as well as all other tables of the sample database are in 3NF.)
Table 4 The employee1 Table