Question

Is is possible to combine the below queries into single, I am basically interested in getting the result given by the last query.

#This is wrong SQL only intended as pseudocode  

SELECT sum(amt) u FROM entry WHERE en_type='some_type' AND num='some_num';
SELECT sum(amt) v FROM entry WHERE en_type='some_other_type' AND num='some_num';
SELECT abal x FROM accounts WHERE num='the_same_num_as_above'; 
SELECT y = x + v - u;

Answer 1

Using subqueries you can do:

SELECT -(SELECT sum(amt) u FROM entry WHERE en_type='some_type' AND num='some_num')
       + (SELECT sum(amt) v FROM entry WHERE en_type='some_type' AND num='some_num')
       + (SELECT abal x FROM accounts WHERE num='the_same_num_as_above') AS y;