What is database redundancy?

Redundancy is when a data / information is stored more than once unnecessarily inside a Database , a rough example:

employee_name and personal_name, each one in a different entity, this is redundancy, because if I have an employee who is a person, I will be storing the same information twice inside the Bank.

But how to avoid redundancies?

Applying the normal forms to the project, usually the first three are enough (1FN, 2FN and 3FN)

It has already been said that redundancy is having the same information in two different places.

To define a bit better what is the same information in two places take a balance and the movement of the account, there is redundant information there. By movement you have the balance. Is this redundancy bad? No. It gives more performance and simplifies access.

Take the customer data from an invoice issued. It's the same one that is in the customer registry. This is redundant, is it bad? No, it is critical because changing the customer data can not change the invoice issued.

It has a number of situations that redundancy is part of the model, even to give more robustness and reliability, in addition to the issues already cited.

Redundancy brings the problem that some coordination is needed to maintain consistency. It should be avoided when it is not needed, so there is normalization .