Should a table connecting two tables have its own ID?

Question

I have two tables:

First
------
id
name

Second
------
id
name

And another table connecting the first two:

Third
------
first_id
second_id

The third table is there only to resolve the M:N issue. Should it have its own ID?

Answer 1

If the table only contains the two foreign keys, there is no reason to have an additional key. You won't be using it in any query.

When you join the tables using the connection table, you are making a join against one foreign key at a time, not against both foreign keys at once, so there is no use for another key in the connection table. Example:

select t1.name, t2.name
from First t1
inner join Third t3 on t3.first_id = t1.id -- one foreign key
inner join Second t2 on t2.id = t3.second_id -- the other foreign key

Just make a primary key combining the two foreign keys.

 PRIMARY KEY (first_id, second_id)

Answer 2

No, you normally don't need an id field on the table connecting the two. You can make the primary key for the connecting table (first_id, second_id).