Re: Difference in columns

Mag Gam wrote:

> I am trying to find the difference between the size column. So the
> desired output would be
>
> ts | size| Diff
> -------------------+-----+------
> 2002-03-16 | 11 | 0
>
> 2002-03-17 | 15 | 4
> 2002-03-18 | 18 | 3
> 2002-03-19 | 12 | -6
>
>
> I need the first column to be 0, since it will be 11-11. The second
> colum is 15-11. The third column is 18-15. The fourth column is 12-18.
>
> Any thoughts about this?

Here's one way to do this with PL/PgSQL. It's probably not the most
efficient, but it does work. For this code to be safe `size' must never
be NULL and `ts' must be unique across all records in the input set.

CREATE OR REPLACE FUNCTION x_diff(
OUT ts TIMESTAMP,
OUT size INTEGER,
OUT diff INTEGER)
RETURNS SETOF record AS $$
DECLARE
cur_x x;
last_size INTEGER := null;
BEGIN
FOR cur_x IN SELECT * FROM x ORDER BY ts ASC LOOP
ts := cur_x.ts;
size := cur_x.size;
IF last_size IS NULL THEN
-- First record in set has diff `0' because the differences
-- are defined against the previous, rather than next,
-- record.
diff := 0;
ELSE
diff := cur_x.size - last_size;
END IF;
last_size := cur_x.size;
RETURN NEXT;
END LOOP;
RETURN;
END;
$$ LANGUAGE 'plpgsql' STRICT;

If you need to constrain the range of values processed that's not too
tricky - either feed the function a refcursor for a query result set to
iterate over, or pass it parameters to constrain the query with a WHERE
clause. The former is more flexible, the latter is easier to use.

--
Craig Ringer