Redundancy means having multiple copies of same data in the database. This problem arises when a database is not normalized. Suppose a table of student details attributes are: student Id, student name, college name, college rank, course opted.
As it can be observed that values of attribute college name, college rank, course is being repeated which can lead to problems. Problems caused due to redundancy are: Insertion anomaly, Deletion anomaly, and Updation anomaly.
- Insertion Anomaly –
If a student detail has to be inserted whose course is not being decided yet then insertion will not be possible till the time course is decided for student.
This problem happens when the insertion of a data record is not possible without adding some additional unrelated data to the record.
- Deletion Anomaly –
If the details of students in this table is deleted then the details of college will also get deleted which should not occur by common sense.
This anomaly happens when deletion of a data record results in losing some unrelated information that was stored as part of the record that was deleted from a table.
- Updation Anomaly –
Suppose if the rank of the college changes then changes will have to be all over the database which will be time-consuming and computationally costly.
If updation do not occur at all places then database will be in inconsistent state.
Reference – practice.geeksforgeeks.org