What should you not store in a database?

Answer 1

You should not duplicate any data. A properly designed database will reduce the amount of duplication. So once it is in the database once, you should not need to enter it again. It wastes space and can lead to inconsistencies in the database.

You should also not store any data that can be calculated from other data that is already in the database. For example, if you have the date of birth stored, there is no need to store the age, as the age can always be calculated from the date of birth. Another reason for not storing the age is that it will need to be updated to keep it accurate. So if you entered your age today, it would still be the same in the database in a year from now or ten years from now. It will not automatically update. If you did not have the date of birth, there would be no way of knowing that the age was accurate, so you should store the date of birth, not the age.

Other things can also be calculated, like if you have the amount of hours and the rate of pay then the actual pay can be calculated, or the cost of a product and the amount sold will enable you to calculate the revenue. So anything that can be calculated from existing data should be left out and just generated through other methods, with formulas on forms or reports and most especially with queries.