RDBPROD: How can I get the row number in Rdb?

Example)

Assume a table with the following content:

Col_A     Col_B

1            2

1            5

1            8

2            1

2            9

3            5

3            6

I need to get the value of Col_B as well as the row number within the set for all rows with a certain value of COL_A.

In Oracle Database I would do this by:

SELECT COL_B, ROWNUM WHERE COL_A = 3;

This would give me the result:

5     1

6     2

I fail to figure out how to do this in Oracle Rdb. Any ideas?