TEST=# create table toto(ti float);
CREATE TABLE
TEST=# create table titi(ot float);
CREATE TABLE
TEST=# insert into toto values(2.2);
INSERT 33577 1
TEST=# insert into titi values(2.2);
INSERT 33578 1
TEST=# select sum(ti)-(select sum(ot) from titi) from toto;
?column?
----------
0
(1 row)
TEST=#drop table titi,toto;
DROP TABLE 2
TEST=#