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Second Normal Form requires that any non-key field be dependent upon the entire key. For example,
consider the StudentCourses table below, where StudentID and
CourseID form a compound primary key.
| StudentID | CourseID | StudentName | CourseLocation | Grade |
| 12345 | 3100 | April | Math Building | A |
| 12345 | 1300 | April | Science Building | B |
The
Student Name field does not depend at all on CourseID, but only on
Student ID. CourseLocation has be dependency on StudentID, but only
on CourseID.
This data should be split into three tables as follows:
Students Table
| StudentID | Name |
| 12345 | April |
Courses Table
| CourseID | CourseLocation |
| 3100 | Math Building |
| 1300 | Science Building |
Student Courses Table
| StudentID | CourseID | Grade |
| 12345 | 3100 | A |
| 12345 | 1300 | B |
In
this example, grade was the only field dependent on the combination
of StudentID and CourseID.
Let’s
suppose that in the first table design, the first row of data was
entered with a StudentName of Aprok, a simple typo. Now, suppose the
following SQL is run.
Delete from StudentCourses where StudentName=”April”
The
erroneous “Aprok” row will not be deleted. However, in the final
design, using the following SQL:
Delete From StudentCourses where StudentID=12345 |
will
delete every course that April was in by using the ID.
Original date of publication, 02/21/2001
