Don’t repeat your columns. Avoid this:
2. Second Normal Form (2NF): No partial dependencies on a concatenated key
This is a complex way of saying that if a column isn’t intrinsically related to the entire primary key,
then you should break out the primary key into different tables.
Example:
Consider OrderDate.
It is conceptually part of an order. An order always occurs at some time.
But is an OrderDate related to an Item? Not really.
You may be saying, “but items are part of an order!”, and you would be right.
But that’s not what I’m getting at. OrderDate is independent of the item itself.
Look at another way:
in the table above the OrderDate will always be the same for a given OrderId
regardless of the value of the ItemId column.
This means data duplication, which is denormalization.
ItemId1, 2, … should be split out into relational tables.
Here’s how we correct the problem:
Here is an excellent line from the article,
“All we are trying to establish here is
whether a particular order on a particular date relies on a particular item.”
3. Third Normal Form (3NF): No dependencies on non-key attributes
2NF covers the case of multi-column primary keys.
3NF is meant to cover single column keys.
Simply stated, pull out columns that don’t directly relate to the subject of the row (the primary key), and put them in their own table.
Example:
Customer information could be the subject of its own table.
Pull out customer name and other customer fields into another table,
and then put a Customer foreign key into Orders.
Sumber:
* http://www.andrewrollins.com/2009/08/11/database-normalization-first-second-and-third-normal-forms/