Can I make sure two columns dont have the same value

If I have a table that looks like this

CREATE TABLE foo (
   id INT NOT NULL AUTO_INCREMENT,
   aa INT NOT NULL,
   bb INT NOT NULL,
   PRIMARY KEY (id),
   UNIQUE KEY (aa, bb),
   CONSTRAINT aa_ref FOREIGN KEY (aa) REFERENCES bar (id),
   CONSTRAINT bb_ref FOREIGN KEY (bb) REFERENCES bar (id)
)

Is there a way to make sure that aa != bb besides using application level logic or forcing a trigger to fail on BEFORE INSERT?

No, you can't. In most DBMS (Postgres, SQL-Server, Oracle, DB2 and many others), you can just add a CHECK constraint:

ALTER TABLE foo 
  ADD CONSTRAINT aa_cannot_be_equal_to_bb_CHK
    CHECK (aa <> bb) ;

I don't see any way to have this in MySQL, using only referential constraints. Besides triggers, you could allow the two columns to have equal values and simply ignore the rows by accessing the table always through a view:

CREATE VIEW foo_correct AS
SELECT id, aa, bb
FROM foo
WHERE aa <> bb ;

Alternatively, you can restrict Insert and Update operations through (stored) procedures that take care of the constraint and not allow data to be inserted (or changed) that do not satisfy it.

StackExchange Database Administrators Q#39885, answer score: 11