Normalization Basics – 1NF 2NF and 3NF

Posted: June 7, 2008 in Uncategorized
Tags: , ,

Description of Normalization

loadTOCNode(2, ‘moreinformation’); Normalization is the process of organizing data in a database. This 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.

Redundant data wastes disk space and creates maintenance problems. If data that exists in more than one place must be changed, the data must be changed in exactly the same way in all locations. A customer address change is much easier to implement if that data is stored only in the Customers table and nowhere else in the database.

What is an “inconsistent dependency”? While it is intuitive for a user to look in the Customers table for the address of a particular customer, it may not make sense to look there for the salary of the employee who calls on that customer. The employee’s salary is related to, or dependent on, the employee and thus should be moved to the Employees table. Inconsistent dependencies can make data difficult to access because the path to find the data may be missing or broken.

There are a few rules for database normalization. Each rule is called a “normal form.” If the first rule is observed, the database is said to be in “first normal form.” If the first three rules are observed, the database is considered to be in “third normal form.” Although other levels of normalization are possible, third normal form is considered the highest level necessary for most applications.

As with many formal rules and specifications, real world scenarios do not always allow for perfect compliance. In general, normalization requires additional tables and some customers find this cumbersome. If you decide to violate one of the first three rules of normalization, make sure that your application anticipates any problems that could occur, such as redundant data and inconsistent dependencies.

The following descriptions include examples.

First Normal Form

loadTOCNode(2, ‘moreinformation’);

Eliminate repeating groups in individual tables.
Create a separate table for each set of related data.
Identify each set of related data with a primary key.

Do not use multiple fields in a single table to store similar data. For example, to track an inventory item that may come from two possible sources, an inventory record may contain fields for Vendor Code 1 and Vendor Code 2.

What happens when you add a third vendor? Adding a field is not the answer; it requires program and table modifications and does not smoothly accommodate a dynamic number of vendors. Instead, place all vendor information in a separate table called Vendors, then link inventory to vendors with an item number key, or vendors to inventory with a vendor code key.

Second Normal Form

loadTOCNode(2, ‘moreinformation’);

Create separate tables for sets of values that apply to multiple records.
Relate these tables with a foreign key.

Records should not depend on anything other than a table’s primary key (a compound key, if necessary). For example, consider a customer’s address in an accounting system. The address is needed by the Customers table, but also by the Orders, Shipping, Invoices, Accounts Receivable, and Collections tables. Instead of storing the customer’s address as a separate entry in each of these tables, store it in one place, either in the Customers table or in a separate Addresses table.

Third Normal Form

loadTOCNode(2, ‘moreinformation’);

Eliminate fields that do not depend on the key.

Values in a record that are not part of that record’s key do not belong in the table. In general, any time the contents of a group of fields may apply to more than a single record in the table, consider placing those fields in a separate table.

For example, in an Employee Recruitment table, a candidate’s university name and address may be included. But you need a complete list of universities for group mailings. If university information is stored in the Candidates table, there is no way to list universities with no current candidates. Create a separate Universities table and link it to the Candidates table with a university code key.

EXCEPTION: Adhering to the third normal form, while theoretically desirable, is not always practical. If you have a Customers table and you want to eliminate all possible interfield dependencies, you must create separate tables for cities, ZIP codes, sales representatives, customer classes, and any other factor that may be duplicated in multiple records. In theory, normalization is worth pursing. However, many small tables may degrade performance or exceed open file and memory capacities.

It may be more feasible to apply third normal form only to data that changes frequently. If some dependent fields remain, design your application to require the user to verify all related fields when any one is changed.

Normalizing an Example Table

loadTOCNode(2, ‘moreinformation’); These steps demonstrate the process of normalizing a fictitious student table.

1. Unnormalized table:

Student# Advisor Adv-Room Class1 Class2 Class3
1022 Jones 412 101-07 143-01 159-02
4123 Smith 216 201-01 211-02 214-01
2. First Normal Form: No Repeating Groups

Tables should have only two dimensions. Since one student has several classes, these classes should be listed in a separate table. Fields Class1, Class2, and Class3 in the above records are indications of design trouble.

Spreadsheets often use the third dimension, but tables should not. Another way to look at this problem is with a one-to-many relationship, do not put the one side and the many side in the same table. Instead, create another table in first normal form by eliminating the repeating group (Class#), as shown below:

Student# Advisor Adv-Room Class#
1022 Jones 412 101-07
1022 Jones 412 143-01
1022 Jones 412 159-02
4123 Smith 216 201-01
4123 Smith 216 211-02
4123 Smith 216 214-01
3. Second Normal Form: Eliminate Redundant Data

Note the multiple Class# values for each Student# value in the above table. Class# is not functionally dependent on Student# (primary key), so this relationship is not in second normal form.

The following two tables demonstrate second normal form:

Students:

Student# Advisor Adv-Room
1022 Jones 412
4123 Smith 216

Registration:

Student# Class#
1022 101-07
1022 143-01
1022 159-02
4123 201-01
4123 211-02
4123 214-01
4.

In the last example, Adv-Room (the advisor’s office number) is functionally dependent on the Advisor attribute. The solution is to move that attribute from the Students table to the Faculty table, as shown below:

Students:

Student# Advisor
1022 Jones
4123 Smith

Faculty:

Name Room Dept
Jones 412 42
Smith 216 42

Source – http://support.microsoft.com/kb/283878

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Comments
  1. Swapan says:

    Good example !!

  2. vikram says:

    i dont understand 3nf.can u give me a clear idea on 3nf in my mail.

  3. sameer says:

    Gud work

  4. ngalande says:

    what about if a no student# was assigned any class? how can we normalize that one?

  5. Rey Anne says:

    A very good example,,, can u give some example of BCNF,4NF,5NF,6NF in my email add plz,,,thank you

  6. Rey Anne says:

    Thank you for this site

  7. farhan says:

    hi,can u help me for do this question??plz,,

    (a) The university keeps track of each student’s name (SNAME), student number (SNUM), social security number (SSSN), current address (SCADDR) and phone (SCPHONE), permanent address (SPADDR) and phone (SPPHONE), birthdate (BDATE), sex (SEX), class (CLASS) (freshman, sophomore, …, graduate), major department (MAJORDEPTCODE), minor department (MINORDEPTCODE) (if any), and degree program (PROG) (B.A., B.S., …, Ph.D.). Both SSN and student number have unique values for each student.

    (b) Each department is described by a name (DEPTNAME), department code (DEPTCODE), office number (DEPTOFFICE), office phone (DEPTPHONE), and college (DEPTCOLLEGE). Both name and code have unique values for each department.

    (c) Each course has a course name (CNAME), description (CDESC), code number (CNUM), number of semester hours (CREDIT), level (LEVEL), and offering department (CDEPT). The value of code number is unique for each course.

    (d) Each section has an instructor (INSTUCTORNAME), semester (SEMESTER), year (YEAR), course (SECCOURSE), and section number (SECNUM). Section numbers distinguish different sections of the same course that are taught during the same semester/year; its values are 1, 2, 3, …; up to the number of sections taught during each semester.

    (e) A grade record refers to a student (Ssn), refers to a particular section, and grade (GRADE).

    Design a relational database schema for this database application. First show all the functional dependencies that should hold among the attributes. Then, design relation schemas for the database that are each in 3NF or BCNF.. Specify the key attributes of each relation. Note any unspecified requirements, and make appropriate assumptions to make the specification complete.

    actually i juz cant do this question “design relation schemas for the database that are each in 3NF or BCNF”

    plz help me ASAP

  8. Ramesh says:

    Any1 plz tel how the dept num 42 comes

  9. danilo says:

    thank you for this site this very helpful

  10. Michael says:

    @ngalande – great point, I was wondering the same thing, and figure they would have to have another table, such as ClassList, with fields ClassList.Class#, ClassList.ClassName, ClassList.ClassDescription

  11. bidya says:

    This is very helpful us .thanx for this…..

  12. Hyonel says:

    this is my assignment hahahaha tnx

  13. I have search a lot and a lot in the internet to get such easy explanation !! and I just found it the easiest and clean example it really help me .. Thank you so much ..

  14. Arshyan says:

    2nd NF is completely wrong, as class# and Advisor is dependent on student but Adv-room is not ..

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