Numeração por blocos

Gostaria de saber se tem como fazer o seguinte:

Tenho um Select (Select contrato, parcela, valor from parcela where dt_rcb >= '01-may-2008' and dt_rcb <= '31-may-2008') que retorna algo parecido com:

a456852 --- 45 --- 78.23

a456852 --- 44 --- 78.23

a456852 --- 46 --- 78.23

a456852 --- 48 --- 78.23

a456852 --- 36 --- 78.23

a456852 --- 21 --- 78.23

b875556 --- 58 --- 96.85

b875556 --- 56 --- 96.85

b875556 --- 57 --- 96.85

b875556 --- 21 --- 96.85

Gostaria de saber como gerar uma consulta que numerasse os blocos como no exemplo abaixo

a456852 --- 45 --- 78.23 --- 1

a456852 --- 44 --- 78.23 --- 2

a456852 --- 46 --- 78.23 --- 3

a456852 --- 48 --- 78.23 --- 4

a456852 --- 36 --- 78.23 --- 5

a456852 --- 21 --- 78.23 --- 6

b875556 --- 58 --- 96.85 --- 1

b875556 --- 56 --- 96.85 --- 2

b875556 --- 57 --- 96.85 --- 3

b875556 --- 21 --- 96.85 --- 4

Quem puder me ajudar eu seria eternamente grata

Valeu