Insertion Anomaly

I am learning insertion anomaly from here. Following data is written in it,

Insert Anomalies

An Insert Anomaly occurs when certain attributes cannot be inserted into the database without the presence of other attributes. For example this is the converse of delete anomaly - we can't add a new course unless we have at least one student enrolled on the course.

StudentNum CourseNum Student Name Address Course
S21 9201 Jones Edinburgh Accounts
S21 9267 Jones Edinburgh Accounts
S24 9267 Smith Glasgow physics
S30 9201 Richards Manchester Computing
S30 9322 Richards Manchester Maths

I am stuck understanding the concept. Why would we need a student to be enrolled into the course for it to exist?

Thanks in advance

3 Answers

This means that the schema is not normalized, i.e. now you have the information about a course in table Student.

So in order to insert course details, you need to provide the student's details as well.

There are different forms of normalization you need to read about, but in this example the right path to solve this anomaly most likely would be to create three tables i.e. strong entity types Student, Course, and an associative entity type linking table StudentCourse (possibly called a Registration or a Grade) which will allow you to store Student and Course data without duplicates and anomalies, as well as assign many courses to many students.

You can read through normalization examples in the following link, it will give you a better idea:

The example assumes that studentnum and coursenum form a composite primary key implementing the integrity rule that a student cannot be enrolled in the same course more than once, i.e. the combination is unqiue. Therefore attempting to add a course record requires a studentnum as well. To avoid this situation while still maintaining the integrity rule the composite key is implemented in an associative entity and course and students are in separate entities.

Insertion anomaly: According to the example taken above where in only one table is there with all the attributes. Now I want to introduce new course then I must be knowing about the student id (student who wants to study that course) which is a primary key and can't be empty. According to the table taken you must know primary key(student id). If you want to enter other fields, but right now we have only new course details without student being registered. So, you cannot just enter new course and leave primary key empty. This is insertion anomaly.