Unique key with NULLs

A fundamental property of a unique key is that it must be unique. Making part of that key Nullable destroys this property.

There are two possible solutions to your problem:

• One way, the wrong way, would be to use some magic date to represent unknown. This just gets you past the DBMS "problem" but does not solve the problem in a logical sense. Expect problems with two "John Smith" entries having unknown dates of birth. Are these guys one and the same or are they unique individuals? If you know they are different then you are back to the same old problem - your Unique Key just isn't unique. Don't even think about assigning a whole range of magic dates to represent "unknown" - this is truly the road to hell.

• A better way is to create an EmployeeId attribute as a surrogate key. This is just an arbitrary identifier that you assign to individuals that you know are unique. This identifier is often just an integer value. Then create an Employee table to relate the EmployeeId (unique, non-nullable key) to what you believe are the dependant attributers, in this case Name and Date of Birth (any of which may be nullable). Use the EmployeeId surrogate key everywhere that you previously used the Name/Date-of-Birth. This adds a new table to your system but solves the problem of unknown values in a robust manner.

I think MySQL does it right here. Some other databases (for example Microsoft SQL Server) treat NULL as a value that can only be inserted once into a UNIQUE column, but personally I find this to be strange and unexpected behaviour.

However since this is what you want, you can use some "magic" value instead of NULL, such as a date a long time in the past