Unique key with NULLs

Question

This question requires some hypothetical background. Let\'s consider an employee table that has columns name, date_of_birth, tit

Answer 1

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