| I understand NULL very well, and that's not the only use for a NULL. NULL also means "value does not exist", not just "value is unknown". For example if a student is not in a class, put NULL in the class id. NULL is perfectly valid data, and is not a replacement for a programming bug. And with mysql if your salary field is defined as accepting NULL then you will get a NULL in there. And to use your example if the field accepts NULL, you would also have no way of knowing if the salary was not negotiated vs a programming bug. If you want to argue the insert should fail, then fine, no problem. (And MySQL can do that.) But arguing that putting in NULL is better (in a field that does not accept NULL), is simply wrong. I'll say it again: NULL is not a replacement for a programming bug - NULL is valid data, and should not be used to find programming errors. |
I think you mean "don't insert a row in the student_class table, which is a many-to-many join between student and class".
As a general rule of thumb, if your data schema requires NULLs for things like that, then your schema is wrong, for most of the reasons that people are trying to point out. NULLs are the absence of data, and should really only be used for exceptional circumstances - hence the reason that silently inserting NULLs into NOT NULL fields is a Bad Thing(tm).