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

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