select a_b,
sum(value)

from
table1

where
date = 20101112
group_by
a_b

I get 2 rows as results like

a 125436.00
b 874563.00

I want to create another row with the totaling a and b showing like

a 125436.00
b 874563.00
tot 998019.00

If you are using pure SQL and not some reporting tool, you will need to do a UNION ALL (two different queries).

Try GROUP BY ROLLUP (a_b)