Describe the third normal form?

Interview Candidate
Aug 24th, 2005
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mdr.mohan rao

Jun 20th, 2005

A table is said to be 3nf if it is already in 2nf and every non-key column is non transitively dependent on primary key.

Kevin B

Jun 24th, 2005

An entity is in the third normal form if it is in the second normal form and all of its attributes are not transitively dependent on the primary key. Transitive dependence means that descriptor key attributes depend not only on the whole primary key, but also on other descriptor key attributes that, in turn, depend on the primary key. In SQL terms, the third normal form means that no column within a table is dependent on a descriptor column that, in turn, depends on the primary key.

For 3NF, first, the table must be in 2NF, plus, we want to make sure that the non-key fields are dependent upon ONLY the PK, and not other non-key fields for its existence. This is very similar to to 2NF, except that now you are comparing the non-key fields to OTHER non-key fields. After all, we know that the relationship to the PK is good, because we established that in 2NF.

Sriharsha Hanumanth

Aug 23rd, 2005

An unnormalized relation is in 1NF If it does not contain any repeating groups.
A 1NF relation is in 2NF if it does not contain any partial dependencies.
A 2NF relation is in 3NF if every determinant is a candidate key.