Question

I understand the concept of database normalization, but always have a hard time explaining it in plain English – especially for a job interview. I have read the wikipedia post, but still find it hard to explain the concept to non-developers. “Design a database in a way not to get duplicated data” is the first thing that comes to mind.

Does anyone has a nice way to explain the concept of database normalization in plain English? And what are some nice examples to show the differences between first, second and third normal forms?

Say you go to a job interview and the person asks: Explain the concept of normalization and how would go about designing a normalized database.

What key points are the interviewers looking for?

Answer 1

Well, if I had to explain it to my wife it would have been something like that:

The main idea is to avoid duplication of large data.

Let’s take a look at a list of people and the country they came from. Instead of holding the name of the country which can be as long as “Bosnia & Herzegovina” for every person, we simply hold a number that references a table of countries. So instead of holding 100 “Bosnia & Herzegovina”s, we hold 100 #45. Now in the future, as often happens with Balkan countries, they split to two countries: Bosnia and Herzegovina, I will have to change it only in one place. well, sort of.

Now, to explain 2NF, I would have changed the example, and let’s assume that we hold the list of countries every person visited.
Instead of holding a table like:

Person   CountryVisited   AnotherInformation   D.O.B.
Faruz    USA              Blah Blah            1/1/2000
Faruz    Canada           Blah Blah            1/1/2000

I would have created three tables, one table with the list of countries, one table with the list of persons and another table to connect them both. That gives me the most freedom I can get changing person’s information or country information. This enables me to “remove duplicate rows” as normalization expects.