If I have 6 fields in a table, where one of them is the id of "x" table so that it becomes the primary key e.g. "x_id" and then have a, b, c, d fields which are also id fields (a_id, b_id, c_id, d_id) to form a composite key, is this ok?

And the next question is how would I code up the MySQL?

Is it allowed to join a composite key field e.g. "a_id" (foreign key) in the child table to the primary key "a_id" of the parent table in a one-to-many relationship?