Can you enforce uniqueness in MySQL based on column in foreign table? I am refactoring one of my database tables and I've come up with an issue.

Specifically, I have a table with four columns. We'll call the first three my.a, my.b and my.c.  I have a uniqueness constraint on the combination of these three columns.  Column my.d is a foreign key to an auto-incremented ID column in another table foreign.

I realized that this other table also has a column foreign.a and both column as are complete duplicates, i.e., if you join the tables together on d, my.a will always equal foreign.a.  Apparently this is part of the requirements.

I would like to remove column my.a from my table, but that will remove the uniqueness constraint.  Is there a way to somehow set up a uniqueness constraint on the combination of foreign.a, my.b, my.c?  Or am I better off just leaving the duplicate column in place?
You cannot directly create a unique constraint across tables. I say directly because you could come up with some indirect scheme; what comes to my mind would be using triggers to insert into a third table that has the unique constraint. But that's a lot of extra work compared to the obvious solution, which is to keep the column "a" in the "my" table.