In relational database design, there is a concept of database normalization or simply normalization, which is a process of organizing columns (attributes) and tables (relations) to reduce data redundancy and improve data integrity. Wikipedia
What is an explanation based on examples about what 1NF, 2NF, 3NF, and 3.5NF (Boyce-Codd BCNF) mean?
1NF is the most basic of normal forms - each cell in a table must contain only one piece of information, and there can be no duplicate rows.
2NF and 3NF are all about being dependent on the primary key. Recall that a primary key can be made up of multiple columns. As Chris said in his response:
The data depends on the key [1NF], the whole key [2NF] and nothing but the key [3NF] (so help me Codd).
Say you have a table containing courses that are taken in a certain semester, and you have the following data:
|-----Primary Key----| uh oh |
V
CourseID | SemesterID | #Places | Course Name |
------------------------------------------------|
IT101 | 2009-1 | 100 | Programming |
IT101 | 2009-2 | 100 | Programming |
IT102 | 2009-1 | 200 | Databases |
IT102 | 2010-1 | 150 | Databases |
IT103 | 2009-2 | 120 | Web Design |
This is not in 2NF, because the fourth column does not rely upon the entire key - but only a part of it. The course name is dependent on the Course's ID, but has nothing to do with which semester it's taken in. Thus, as you can see, we have duplicate information - several rows telling us that IT101 is programming, and IT102 is Databases. So we fix that by moving the course name into another table, where CourseID is the ENTIRE key.
Primary Key |
CourseID | Course Name |
---------------------------|
IT101 | Programming |
IT102 | Databases |
IT103 | Web Design |
No redundancy!
Okay, so let's say we also add the name of the teacher of the course, and some details about them, into the RDBMS:
|-----Primary Key----| uh oh |