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

This is a 1 to zero-to-many relationship. I have used this many times with SQL Server. I believe it is possible to do this with MySQL as well.

I prefer to avoid NULLs in my databases because of issues related to data aggregation so, depending on my design, I put an UNKNOWN row in the lookup table.