From: | Craig Ringer <craig(at)postnewspapers(dot)com(dot)au> |
---|---|
To: | Mag Gam <magawake(at)gmail(dot)com> |
Cc: | pgsql-sql(at)postgresql(dot)org |
Subject: | Re: Difference in columns |
Date: | 2008-05-11 19:58:56 |
Message-ID: | 48275000.90002@postnewspapers.com.au |
Views: | Raw Message | Whole Thread | Download mbox | Resend email |
Thread: | |
Lists: | pgsql-sql |
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
From | Date | Subject | |
---|---|---|---|
Next Message | Mag Gam | 2008-05-12 00:58:28 | Re: Difference in columns |
Previous Message | Tom Lane | 2008-05-11 18:58:02 | Re: Difference in columns |