WebSphere Application Server - IBM Redbooks

2.5.2 Indexes
Third normal form
In the third normal form, each non-key column is dependent upon the key column
as in the second normal form, but is also independent of other non-key columns.
In Table 2-1 we can see that the DeptName column is dependent on the DeptNo
column. This table does not conform to the third normal form. If we update
DeptName for one employee, it would not automatically update the DeptName
column for other employees belonging to the same department
Table 2-1 Table not in third normal form
EMPNO(Primary Key) LastName DeptNo DeptName
001 Brown 01 Support
002 Smith 02 Sales
To normalize this data to conform to the third normal form, we would create two
tables. The first table, Table 2-2, contains the relation between the employee and
the department number. The second table, Table 2-3, contains the relation
between the department number and the department name. An update to the
department name only needs to be done once.
Table 2-2 Table in third normal form
EMPNO(Primary Key) LastName DeptNo
001 Brown 01
002 Smith 02
Table 2-3 Table in third normal form
DeptNo(Primary Key) DeptName
01 Support
02 Sales
One consideration when normalizing tables is that the more you normalize, the
better it results in insert and update performance. However, it can slow down the
query response time.
Indexes are used to improve query performance. They provide a faster
mechanism to search the database table for a particular column value(s) and
avoid expensive and slower table scans. Columns of a table that are often used
in search clauses are prime candidates for indexes. The downside of indexes is
that they take up disk space and slow execution of insert and update statements.
38 IBM WebSphere Application Server - Express V5.0.2 Developer Handbook