Sunday, May 1, 2011

First Normal Form (1NF)

A table is considered to be in 1NF if:
  1. There is no top-to-bottom ordering to the rows
  2. There is no left-to-right ordering to the columns
  3. There are no duplicate rows
  4. Every row-and-column intersection contains exactly one value*
  5. All columns are regular (no hidden components)
A table without a unique key (meaning that duplicate rows could exist) or a table with a nullable column (a violation of condition 4) would not be in 1NF.


*Condition 4, which does not allow repeating groups, is thought by many to be the defining feature of 1NF.

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