mysql qerry

I have a table with three columns: id (index), subid (index), code (string)

for every id there may be several subid's but there is at least one. For every id and given subid the code should be the same. ie there should be a one to one relationship between codeid and code for a given id. Should have done this in two tables of course. What I need is a query that shows the rows where this is not true.