might be able to help you with this but need a few questions answered.
1) I am assuming that ID+Cat_Name is unique, in other words, there cannot be two 101 A1 entries. is this correct?
2) what are specifications for the Cat_Name field? Is it always exactly two characters(one letter and then one number) or can it be one letter followed by any integer, etc.
3) you said 3-5 rows returned. Do you want that number random as well, or can it be the same number of rows returned everytime as long as that number is between 3-5?