To this point, you’ve been working mostly with straightforward data values from the tables. However, you can also recalculate values to give you a form that’s more suitable to a particular situation.
There are a number of interrelated principles involving the type of values you keep in SQL table columns:
Values should never be repeated.
This includes variations of the same value, such as the height in centimeters and the height in inches or the date of birth and the age.
All columns should be independent ...
