How to create sub report

Question

Hi,
i have a database table which contains multiple records i mean repeated records
looks like..

unitid-invoiecNo-itemdate
1  2      12-12-2012
1  3  12-12-2012
1  4   12-12-2012
1  5   12-12-2012
2  6       12-12-2012
2   7   12-12-2012
2   8   12-12-2012
2   9   12-12-2012

now i want in this view condition

unit: 1 total:4
invoice-itemdate
2- 12/12/2012
3- 12/12/2012
4- 12/12/2012
5- 12/12/2012
unit: 2 total:4
invoice-itemdate
6- 12/12/2012
7- 12/12/2012
8-   12/12/2012
9- 12/12/2012

means when second unit start it should create header for that unit and then record of that unit...


please help me