Stored procedure in MySQL? Are you sure? As of the 4.x alpha release I didn't think MySQL supported SPs. In fact, I'm 99.99% sure it doesn't.
I'm guessing here but the data types you're using there are all *LOBs (large objects). I think MySQL stores *LOBs in the same way as other RDBMSs, in that the data isn't stored in a row (as it has the potential to be VERY big) but the row stores a reference to the object.
So I'm guessing that's why you can't select distinct. Try changing the data types and see what happens.