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2010年8月1日星期日

FOREIGN KEY Constraint Allow Multi-NULL Values -- Not Mentioned by MySQL 5.0 Reference Manual

Compare to UNIQUE KEY Constraint ( the manual says: For other engines (all but BDB), a UNIQUE index allows multiple NULL values for columns that can contain NULL. ), whether 'allow FOREIGN KEY Constraint to have multi-NULL values' is not mentioned.

By tested on MySQL 5.0 / Ubuntu 8.04, the system says YES, the multi-NULLs are allowed.

I think this should be added to the manual explicitly.

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