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

Question

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.

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Answer 1

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.