sql - How to subtract from previous row result?

Question

I need to subtract first row lot_size + substraction result from currentitems column. If no balance left then it should be 0. bal is how result column should look.

    rowno | location | lot_size | currentitems |  bal   | bal_left  
   -------+----------+----------+--------------+--------+--------
        1 | AB1210   |     1200 |         1000 |   1000 |   200
        2 | AB1220   |     1200 |         1000 |    200 |     0
        3 | AB1230   |     1200 |          500 |      0 |     0

Current approach (using postgresql 9.3.1):

SELECT 
    row_number() over (ORDER BY location) as rowno, 
    location,
    currentitems,
    1200 as lot_size,
    --here should be some case or something
    COALESCE(lag(currentitems) over(ORDER BY location),currentitems) AS bal
FROM foo;

Answer 1

Not sure if that's what you're asking, but here's how you can get next row:

create table a(
  id bigserial primary key,
  val integer
);

select 
  nth_value(val,(row_nr+1)::integer) over () as next_value, 
  val, from 
      (select row_number() over () as row_nr, val from a) t;

And then you can do your cases and subtractions