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Third Normal Form


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A table is in 3rd NF when it is in Second Normal Form and all non-key fields are dependent ONLY on the Primary Key and there is no co-dependence between non-key fields. A table can be put into 3rd NF by removing any co-dependent fields to another table where they are not co-dependent.
Also known as 3NF.
EXAMPLE

Assume a purchase order table with the following structure:
FieldMeaning
PoidPK for this table
PodateDate of this PO
VendoridFK to the vendor table
VendorcityCity where vendor is located
etcOther fields for the po record

Ask yourself this question for each field, "Is there any non-key (primary or alternate) field that this one is dependent on for its value?"

Podate = No, only the PK
Vendorid = No, only the PK
Vendorcity = Yes, while the PK is required the city is also dependent on the Vendorid which is not an alternate key for the PO table.

The solution is to remove the Vendorcity from this table and put it in a table where is dependent only on the PK.
Category Data Modeling Data Normalization Boyce Codd NF
( Topic last updated: 2002.10.18 07:42:08 PM )