How to merge multiple rows with same column value into one row in sql db2 and join with another table

Question

I have this table and I want to combine rows that have the same REFNO into one row

This is the result that I want, the ID follows the upper row so if I merge row 2 and 3 the new row will have an id of 2

I want to join this table with another table according to its REFNO. This is the other table