sql - reset row_number for different groups in SELECT -

i need add different groups of row counts selected data below:

no. date co.
1 15/5/7 a
2 15/5/8 a
3 15/5/9 a
1 15/5/7 b
2 15/5/8 b
3 15/5/9 b

my current code :

select row_number() over(order date) no,date, co.   table    order date asc, co.   

resulting this:

no. date co.
1 15/5/7 a
2 15/5/8 a
3 15/5/9 a
4 15/5/7 b
5 15/5/8 b
6 15/5/9 b

how achieve this?

make partition co column

select    row_number() over(partition co order date) no,   date,    co table order co, date asc