Denormailzed columns

Hi,

what is meant by denormalized columns?
When it's required/when we'll create it & how the values for these columns are obtained?

Can anyone explain this concept with an example?

Thanks

I'm guessing you mean to say denormalized table. An example would be for phone number. Any person can have a home, cell, work, and fax numbers. Given that the is a one to many relationship between person and phone number, you could store phone number as a separate table from person with a primary key consisting of person id, and phone number type. But doing so will require a join to bring back person and phone number, so instead you can denormalize the table by making 4 extra columns in your person table. One for each type of number that you track.