Database Normalization Page 2

By David Faour (Send Email)
Posted Feb 8, 2008


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

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