How do I create a primary composite key from three tables?

Question

Suppose I have tableA with A.id, tableB with B.id and table C with C.id. Now I want to create a table D that has a primary composite key(A.id, B.id, C.id). How can I do this?

create tableD(

    A.id ..
    B.id ..,
    C.id ..,
    
    primary key (A.id, B.id, C.id),
    foreign key (A.id, B.id, C.id) references ??
)

Answer 1

So you have one table with a composite Primary Key, each element of which references the Primary Key of another table.
Without specifics, let's take a simplistic example of a Student taking a Class taught by a specific Teacher. Each Entity (Student, Class, Teacher) can exist on its own (i.e. has its own table) and the association of the three is a student actually taking a class (OK, I'm ignoring dates in all this!).

create tableA ( id ... primary key ( id ) ) ;
create tableB ( id ... primary key ( id ) ) ;
create tableC ( id ... primary key ( id ) ) ;

create tableD
( a_id ...
, b_id ...
, c_id ...
, foreign key ( a_id ) references tableA ( id ) 
, foreign key ( b_id ) references tableB ( id ) 
, foreign key ( c_id ) references tableC ( id ) 
);