How are ER subtypes modeled in a database?

Question

Sorry if this is too easy, but lately I have found a lot of documentation on ER-modeling, but all this seems to skip the actual implementation, and I just want to clarify.

Is only the second table with a foreign key for the supertype, along with the properties related to the subtype, a subtype? In any case, it will be of great value to me. Will the primary subtype key also be shared with the supertype (will the primary subtype have an external supertype restriction)?

Answer 1

Yes, this is one of three ways to do this.

The second method, and perhaps the simplest one, is simply to have values ​​in the subtypes in the fields of the supertype, which can be zero. More space is required, but it increases performance because fewer queries are required to obtain subtype-specific data.

The third way is to have a table for each type / subtype. This is only effective if you always know the type / subtype of the data you need to look for. It also does not require as much space as a second.