How to make a foreign key unique, but at the same time, so that it can be NULL?

Question

MySQL

How to make a foreign key unique, but at the same time, so that it can be NULL?
PS I reformulated my question in a different way: How to build the architecture of tables in my case? .

Answer 1

In MySQL, NULL != NULL, so there can be any number of NULLs in a column with a UNIQUE index.

Answer 2

No, these are conflicting requirements (mutually exclusive)

Answer 3

On sql it is hardly realizable. But you can try, if possible, just do a check, for example in php, before writing
. That is, when you want to insert a record, you first need to check in the database, then if the key is not set, then write it down, otherwise give an error