Describe/define - when to de-normalization data?

Question

Recently came across an interview question where it was asked - When should we de-normalize the data? and what is the benefit of that?

Muhammad Arsalan · Answer

Database normalization and de-normalization don't follow any particular set of rules it actually depends on conditions when to normalize and when to go the other way. Before describing normalization I assume you know well about data redundancy. Normalization actually reduces data redundancy but it creates an over head as it increases number of tables. By this increase in number of tables a query may have to go through many tables collecting data. On the other hand in an un-normalized database there are very few tables which increases risk of data redundancy. But at the same time a query dont have to move around so many tables as in normalized case. So it all depends on how much load we can afford. It should be a balance between data redundancy and database overhead.

Tapan Patel · Answer

Well. Thanks for the information but this is something everyone would know by reading some stuff on the internet. What I would like to know is the specific examples -real time where you won't find normalization.