On the paradigm of relational database

A joins is not allowed to have multiple values: such as the class inside so save, 1 classes, 2 classes, 3 classes is a string

such as the answer to save a,b,c,d these four options

But my soft teacher didn't mention this paradigm, nor did the Web teacher. , I didn't have a classmate, and I didn't react at first.

Not that it's not possible.

It's just you. According to relational database theory: Reduces the logical and physical independence of data

That is, the data definition is saved in the program. That's what it is.

On the paradigm of relational database