Sub Serial Number Generation in SSRS

Question

How to generate sub serial number in ssrs reports..

 

Sr.NO   Sub.Sr.No   Dept

1                               A

                  1.1          a

                  1.2          b

                  1.3          c

2                               B

                  2.1          a

                  2.2          b

3                               C

                  3.1          a

                  3.2          b