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Basic SQL structure

Now that we see the basics of SQL, let’s take a look at the normalization steps, so that we can understand table structure.

Un-normalized Form (0NF)

Essentially, an un-normalized relation is a relation that contains repeating values. An un-normalized relation can also contain relations nested within other relations, as well as all kinds of transitive dependencies.  Sometimes un-normalized relations are signified by 0NF, but an un-normalized relation is not to be confused with a denormalized relation. 

The un-normalized relation is any relation in its raw state, and they commonly contain repeating vales and other characteristics that are not found in denormalized relations.  The process of denormalization is a very deliberate attempt to introduce controlled redundant items into an already normalized form.

First Normal Form (1NF)

In essence, any relation is in first normal form if it does not contain any repeating values. Here, we have taken our relations with repeating values and moved them to separate relations. When the new relations are created, we carry the primary key of the original relation into the new relation. 

We start by creating a primary key to uniquely identify each row in the table.  Any repeating data is moved to a new table.  Finally, create a key for each of the rows of the new table and a reference to that key in the original table.  This is like what we did with the FRIENDS and ADDRESS tables above. First normal form removes repeating data to separate tables.  Each of the new tables may have repeating data that should be moved to another new table.  This process continues until there is no longer redundant data.

Second Normal Form (2NF)

For second normal form, you must start in first normal form.  Then, each column in a table must be dependent on the key for that table, or it should be move to a new table. 

The purpose of the second normal form (2NF) test is to check for partial key dependencies. Partial key dependencies are created when we break off an un-normalized relation into first normal form by carrying the key thereby creating a concatenated key with several data items. The formal definition of second normal form is as follows: 

A relation is in second normal form if and only if the relation is in first normal form and each non-key attribute is fully functionally dependent on the entire concatenated key.


The above book excerpt is from:

Easy Oracle SQL

Get Started Fast writing SQL Reports with SQL*Plus

ISBN 0-9727513-7-8

Col. John Garmany 

http://www.rampant-books.com/book_2005_1_easy_sql.htm

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