If judgment in MySQL

The problem is that there is a table (Tb_class) that specifically holds the class ID and class name

Another table is the Student Information table (TB_STU), the table has a field called ClassID, there is no foreign key association, now you want to flush this table to another table tb_partment, the table ClassID (can be null) associated with the ID in Tb_class, Therefore, it is necessary to judge if the ClassID in Tb_stu is not in Tb_class, then the ClassID in Tb_stu is set to null.

This can be implemented in the DAO layer of Java, but I think this should also be implemented in MySQL, in the Tb_stu data to the memory, or memory in the memory of the data inserted into the tb_partment can be judged by an if, and this judgment is done in MySQL. Now, write down the question first. Because I'm not going to do it now.

If judgment in MySQL