CODEWITHSUNDEEP
postgresqlaggregate-functions
dmz73
dmz73

Reputation: 1608

Using aggregate functions on alias?

I want to make a query where I am computing the difference between two columns. Something like:

 SELECT a,
        b,
        a - b as "diff"
  FROM ...

Now I would like to calculate the stddev of the "diff" column using postgresql built-in stddev aggregate function. How can I achieve this?

Thanks.

EDIT:

The actual query is this:

SELECT tr.date_start,
       tr.date_end,       
       (((CASE when(tourney_summary.val_curr_conv != 0) THEN tourney_summary.val_curr_conv * (tr.amt_won + tr.cnt_bounty * tourney_summary.amt_bounty) ELSE 0.0 END))) AS "amt_won_curr_conv",
          (((CASE when(tourney_summary.val_curr_conv != 0) THEN tourney_summary.val_curr_conv * (tourney_summary.amt_buyin + tourney_summary.amt_fee + tourney_summary.amt_rebuy * tr.cnt_rebuy + tourney_summary.amt_addon * tr.cnt_addon + tourney_summary.amt_bounty) ELSE 0.0 END))) AS "amt_buyin_ttl_curr_conv",
       ((((CASE when(tourney_summary.val_curr_conv != 0) THEN tourney_summary.val_curr_conv * (tr.amt_won + tr.cnt_bounty * tourney_summary.amt_bounty) ELSE 0.0 END))) - (((CASE when(tourney_summary.val_curr_conv != 0) THEN tourney_summary.val_curr_conv * (tourney_summary.amt_buyin + tourney_summary.amt_fee + tourney_summary.amt_rebuy * tr.cnt_rebuy + tourney_summary.amt_addon * tr.cnt_addon + tourney_summary.amt_bounty) ELSE 0.0 END)))) as net_amt_won,
       stddev((((CASE when(tourney_summary.val_curr_conv != 0) THEN tourney_summary.val_curr_conv * (tr.amt_won + tr.cnt_bounty * tourney_summary.amt_bounty) ELSE 0.0 END))) - (((CASE when(tourney_summary.val_curr_conv != 0) THEN tourney_summary.val_curr_conv * (tourney_summary.amt_buyin + tourney_summary.amt_fee + tourney_summary.amt_rebuy * tr.cnt_rebuy + tourney_summary.amt_addon * tr.cnt_addon + tourney_summary.amt_bounty) ELSE 0.0 END)))) as diff_std_dev

FROM tourney_summary,
     tourney_results tr
WHERE 
  tr.id_player=1
  AND tourney_summary.id_tourney = tr.id_tourney
  AND ((tourney_summary.id_gametype = 1)
       AND (((((((tourney_summary.id_table_type IN
                    (SELECT lttt.id_table_type
                     FROM tourney_table_type lttt
                     WHERE lttt.val_seats = 2))))))
             AND (((((tourney_summary.id_table_type IN
                        (SELECT lttt.id_table_type
                         FROM tourney_table_type lttt
                         WHERE position('S' IN lttt.val_speed) > 0))
                     OR (tourney_summary.id_table_type IN
                           (SELECT lttt.id_table_type
                            FROM tourney_table_type lttt
                            WHERE position('H' IN lttt.val_speed) > 0))))))))
       AND ((tourney_summary.date_start >= '2013/08/15 23:00:00')))
GROUP BY tr.date_start,
         tr.date_end,
         tourney_summary.val_curr_conv,
         tr.amt_won,
         tr.cnt_bounty,
         tourney_summary.amt_bounty,
         tourney_summary.amt_buyin,
         tourney_summary.amt_fee,
         tourney_summary.amt_rebuy,
         tr.cnt_rebuy,
         tourney_summary.amt_addon,
         tr.cnt_addon
ORDER BY tr.date_end DESC;

The "a" and "b" expressions (the ones with CASE) are big. And I don't know how to avoid the copy/paste. In any case using stddev on the a-b expression returns a blank column. What am I doing wrong?

Thanks.

Upvotes: 1

Views: 616

Answers (2)

roman
roman

Reputation: 117370

it's also possible to do this with cte

with cte1 as (
   select a, b, a - b as diff
   from ...
)
select
    a, b, diff, stddev(diff) as diff_stddev
from cte1

Upvotes: 2

Craig Ringer
Craig Ringer

Reputation: 324415

You pretty much answer it yourself. Calculate the standard deviation of the difference:

 SELECT a,
        b,
        a - b as "diff",
        stddev(a - b) AS "diff_stddev"
  FROM ...

If a - b is a computationally expensive operation or is in fact a much more complex expression in reality, you can wrap it in a subquery:

 SELECT a, b, "diff", stddev("diff") AS diff_stddev
 FROM (
   SELECT a, b, a - b
   FROM ...
 ) x (a, b, "diff")

x is just a throw-away alias for the subquery table.

Upvotes: 3

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