Foreign keys and NULL in mySQL

Question

Can I have a column in my values table (value) referenced as a foreign key to knownValues table, and let it be NULL whenever needed, like in the example:

Table: valu

Answer 1

Yes it is quite possible to have a NULL in your foreign-key-constrained column. I just tried it. Bear in mind that if you are not using the InnoDB storage engine, your foreign key constraints will be ignored anyway.