grouping the batch in sql server :  have a scenario wherein I have

Id|rank|   date
1 | 7  |07/08/2015
1 | 7  |09/08/2015
1 | 8  |16/08/2015
1 | 8  |17/08/2015
1 | 7  |19/08/2015
1 | 7  |15/08/2015
2 | 7  |01/08/2015
2 | 7  |02/08/2015
2 | 8  |16/08/2015
2 | 8  |17/08/2015
2 | 7  |26/08/2015
2 | 7  |28/08/2015
My desired solution is

1 | 7  |07/08/2015 |1
1 | 7  |09/08/2015 |1
1 | 8  |16/08/2015 |2
1 | 8  |17/08/2015 |2
1 | 7  |19/08/2015 |3
1 | 7  |15/08/2015 |3
2 | 7  |01/08/2015 |4
2 | 7  |02/08/2015 |4
2 | 8  |16/08/2015 |5
2 | 8  |17/08/2015 |5
2 | 7  |26/08/2015 |6
2 | 7  |28/08/2015 |6
i.e for each block of id and rank I want to add the new column and updated the same. 