Normalization is the process of organizing data in a database. It includes creating tables and establishing relationships between those tables according to rules designed both to protect the data and to make the database more flexible by eliminating redundancy and inconsistent dependency.
First normal form (1NF). This is the "basic" level of database normalization, and it generally corresponds to the definition of any database, namely:
It contains two-dimensional tables with rows and columns.
Each column corresponds to a subobject or an attribute of the object represented by the entire table.
Each row represents a unique instance of that subobject or attribute and must be different in some way from any other row (that is, no duplicate rows are possible).
All entries in any column must be of the same kind. For example, in the column labeled "Customer," only customer names or numbers are permitted.
Second normal form (2NF). At this level of normalization, each column in a table that is not a determiner of the contents of another column must itself be a function of the other columns in the table. For example, in a table with three columns containing the customer ID, the product sold and the price of the product when sold, the price would be a function of the customer ID (entitled to a discount) and the specific product. In this instance the data in the third column is said to be dependent upon the data in the first and second columns. This dependency does not occur in the 1NF case.
The column labeled customer ID is considered a primary key because it is a column that uniquely identifies the rows in that table, and it meets the other accepted requirements in standard database management schema: It does not have NULL values and its values won't change over time.
In the example above, the other column headers are considered candidate keys. The attributes of those candidate keys that make them unique are called prime attributes.
Third normal form (3NF). At the second normal form, modifications are still possible because a change to one row in a table may affect data that refers to this information from another table. For example, using the customer table just cited, removing a row describing a customer purchase (because of a return, perhaps) will also remove the fact that the product has a certain price. In the third normal form, these tables would be divided into two tables so that product pricing would be tracked separately.
Extensions of basic normal forms include the domain/key normalized form, in which a key uniquely identifies each row in a table, and the Boyce-Codd normal form (BCNF), which refines and enhances the techniques used in the 3NF to handle some types of anomalies.
Database normalization's ability to avoid or reduce data anomalies, data redundancies and data duplications, while improving data integrity, has made it an important part of the data developer's toolkit for many years. It has been one of the hallmarks of the relational data model.
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