How to obtain algebraic sum of equal lines

Hi all,
Is it possible, given the following table:

cod_rif | titolo | quantity |   cmf   |  u_qq  |  mont   | vend |  sum
---------+--------+----------+---------+--------+---------+------+-------
 26      | aaa    |    -1000 | 6.11098 |  6.176 |  6.1110 | t    | -1000
 7       | aaa    |     2500 |  25.455 |      0 | 60.0897 | f    |  2500
 28      | bbb    |     2700 |   3.862 |    4.6 |  4.1957 | f    |  2700
 3       | bbb    |      500 |   6.057 | 10.129 | 18.7311 | f    |   500
 34      | bbb    |     -700 |       0 |      0 |  0.0000 | t    |  -700
 6       | bbb    |     -500 |   2.703 |  4.757 |  3.7151 | f    |  -500
 30      | ccc    |     5000 |   1.717 |  1.489 |  1.7170 | f    |  5000
 33      | ccc    |    10000 |    4.36 |      0 |  4.3600 | f    | 10000
 11      | ddd    |    -1500 |  10.537 |  4.021 | 20.5815 | t    | -1500
 32      | ddd    |     1500 | 0.55896 |  1.119 |  0.5590 | f    |  1500
(10 rows)

to get a selection whereby the algebraic sum of "quantity" for each equal 'titolo'
is returned?

I tried this query with no result:
psql finanza -c "select * , (sum(quantity)) from  test_t group by cod_rif, titolo, quantity, cmf, u_qq, mont, vend
orderby titolo asc"  

Regards,
    Ennio.


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