Question

As an example,
I have a 3 tables:

School: ID int, Name varchar

Student: ID int, Name varchar

StudentInSchool: StudentID int, SchoolID int

Now the question is whether I should put a column ID int with a primary key on it in StudentInSchool table? If yes, why?

Will it be helpful in indexing?

Any help appreciated.

Answer 1

Personally, I create composite PK (StudentID and SchoolID) on such junction tables. This also ensures uniqueness.

If, however, uniqueness is not required, you’ll have to add an ID column to uniquely identify each row.

Generally speaking, addition of a separate ID column will not help much: very few queries (if any) will actually use this column. As for performance, you can create separate index for each column and you’ll be just fine.