In SQL database tables, why is redundant data generally a bad thing? [closed]

Question

And when is it necessary?

(for redundant data I mean same data in multiple tables)

Answer 1

This is a broad question, but I think it has some succinct answers.

Redundant data is a bad idea because when you modify data (update/insert/delete), then you need to do it in more than one place. This opens up the possibility that the data becomes inconsistent across the database.

The reason redundancy is sometimes necessary is for performance reasons. Often, redundant data is simply easier to work with from a querying perspective.