- ORACLE does not permit NULL values in any of the columns that comprise aprimary 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.
In Oracle, you can’t have null values in Composite primary key columns. I’m not sure about MySQL , SQL server,..
i read somewhere in oracle forums that even if you declare columns which you want to make as composite primary key as nullable. when you are adding primary key constraint, the columns would become not null columns.