Question

I have 2 tables with many to many relation:

student 
(
id int(11) NOT NULL, 
name varchar(255), 
primary key(id)
);

teacher
(
id int(11) NOT NULL, 
name varchar(255), 
primary key(id)
);

and I should do 3 table – student_has_teacher

1. option add id separate field primary key

   student_has_teacher
   (
   id int(11) NOT NULL,
   teacher_id int(11),
   student_id int(11)
   primary key(id)
   );

2. option make 2 fields primary key

   student_has_teacher
   (
   teacher_id int(11),
   student_id int(11),
   primary key(teacher_id,student_id),
   foreign key(teacher_id) references teacher(id),
   foreign key(student_id) references student(id)
   );

What is better option and why?

Thanks

Answer 1

The two options are not equivalent. In option 1 there can be multiple pairings of each teacher and student. In option 2 only 1 row is permitted for each combination of teacher and student.

There is another difference as well. In 1 the student and teacher are nullable. In 2 they aren’t.

On the information given I don’t see why a teacher would need to be paired with the same student more than once. So as a guess I’d say that 2 was more appropriate, but it all depends on the business requirements and you haven’t really given much information to say either way.