Framework relations problem

[TziRi]

I have 3 tables in the framework(A,B,C). I make joins A-B-C, if i will use in my report all 3 tables it will show me that the types are not compatible,but if i run the report with only A-B it will work,B-C work also,so the joins are correct, but when i try to work with info from A and C doesn't work. For my mind this seems impossible, if A-B works and B-C work then it should work also A-C. I connect all of them with the same columns User and Month, so if A-B are the same type and works and B-C are the same type and works ,why A-C doesn't work?
A(0-1)-->(1-n)B(1-1)-->(1-1)C 
A and B are oracle
C is mysql

[Francis aka khayman]

can you give the datatypes of user and month columns for each table?

wild guess is that month does not get converted properly for A-C join. to confirm, remove the month join and see if it works.