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

Of course, there is a possibilities ti have a NULL values in foreign key, but for that you don't get worry about this, I hope you may used InnoDB as database engine to manage the Key constraints. For this case i suggest to use Left Join or Right Join to get rows from DB and Group By can be used for avoid duplication. Please do not use Inner Join.