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What's wrong with nullable columns in composite primary keys?

Asked 2008-12-22T11:33:55.363
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ORACLE does not permit NULL values in any of the columns that comprise a primary key. It appears that the same is true of most other "enterprise-level" systems.

At the same time, most systems also allow unique contraints on nullable columns.

Why is it that unique constraints can have NULLs but primary keys can not? Is there a fundamental logical reason for this, or is this more of a technical limitation?

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A primary key defines a unique identifier for every row in a table: when a table has a primary key, you have a guranteed way to select any row from it.

A unique constraint does not necessarily identify every row; it just specifies that if a row has values in its columns, then they must be unique. This is not sufficient to uniquely identify every row, which is what a primary key must do.

answered 2008-12-22T11:53:50.083

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