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Create Example Table in First Normal Form (DBMS)
If a table has data redundancy and is not properly normalized then it will be difficult to handle and update. If a relation contains composite or multivalued attributes it violates the first normal form.
A table is in 1Nf if the following conditions exist −
- There are only single valued attributes.
- Attribute domain does not change.
- There is a unique name for every attribute or column.
- The order in which data is stored does not matter.
Example
Consider the table given below −
| Id | Name | Course |
|---|---|---|
| 1 | A | C1,C2 |
| 2 | E | C3 |
| 3 | B | C2,C3 |
The above table is in un normalized form, the course attribute is having multivalued attributes which violates 1NF.
Now we represent the above table by creating a new row for each course as −
| Id | Name | Course |
|---|---|---|
| 1 | A | C1 |
| 1 | A | C2 |
| 2 | E | C3 |
| 3 | B | C2 |
| 3 | B | C4 |
The above table contains redundant data due to course, as for each course number we have to repeat all the information of students. So course attributes should be separated from the above table.
We divide or decompose the above table R into two tables which is the concept of normalization −
R1(key, multivalued attribute), R2(R-multivalued attribute)
=>R1(Id, course), R2(Id , name).
Steps to decompose the 1NF table
Place all items that appear in the repeating group in a new table. Find a primary key for each new table produced. Duplicate in a new table the primary key of the table from which the repeating group was extracted or vice versa.
R1
| Id | Course |
|---|---|
| 1 | C1 |
| 1 | C2 |
| 2 | C3 |
| 3 | C2 |
| 3 | C4 |
R2
| Id | Name |
|---|---|
| 1 | A |
| 2 | E |
| 3 | B |
R1 and R2 are in 1NF.
Key of R1 = ID
Key of R2 = (rollno, course)
The above table is now in 1NF because no multivalued attributes are present. But it is not in 2NF because ID-> name is a partial dependency {since name depend on apart of key}
Anomalies
The table also suffers with anomalies as follows −
Insertion anomaly − We cannot insert a new course such as 'C6' to the table unless we have a student who has to take the subject.
Updation anomaly − If we change the course from C2 to C5 we have to make changes in more than one place otherwise the table will be inconsistent.
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