I don't think that MySQL supports full outer join. But you can do this by yourself in custom SQL using two joins and a UNION.
A full outer join keeps all the records from both tables and it is equivalent of combining results from a left outer join and a right outer join then getting rid of the duplicates.
So something like this
select TableX.ID, ColA, ColB from TableX right join TableY on TableX.ID = TableY.ID
Select TableY.ID, ColC, ColD from TableY right join TableX on TableX.ID = TableY.ID
Union will get rid of the duplicates for you.
I hope this helps.
I am not sure why this option is offered in the UI.