To compute sum regarding to a constraint - sql

Question

I'm using PostgreSQL 8.4.
I have the following sql-query:
SELECT p.partner_id,
CASE WHEN pa.currency_id = 1 THEN SUM(amount) ELSE 0 END AS curUsdAmount,
CASE WHEN pa.currency_id = 2 THEN SUM(amount) ELSE 0 END AS curRubAmount,
CASE WHEN pa.currency_id = 3 THEN SUM(amount) ELSE 0 END AS curUahAmount
FROM public.player_account AS pa
JOIN player AS p ON p.id = pa.player_id
WHERE p.partner_id IN (819)
GROUP BY p.partner_id, pa.currency_id
The thing is that query does not what I expected. I realize that, but now I want to understand what exactly that query does. I mean, what SUM will be counted after the query executed. Could you clarify?

Answer 1

I think you have the conditions backwards in the query:
SELECT p.partner_id,
SUM(CASE WHEN pa.currency_id = 1 THEN amount ELSE 0 END) AS curUsdAmount,
SUM(CASE WHEN pa.currency_id = 2 THEN amount ELSE 0 END) AS curRubAmount,
SUM(CASE WHEN pa.currency_id = 3 THEN amount ELSE 0 END) AS curUahAmount
FROM public.player_account pa JOIN
player p
ON p.id = pa.player_id
WHERE p.partner_id IN (819)
GROUP BY p.partner_id;
Note that I also removed currency_id from the group by clause.

Answer 2

Maybe one row per (partner_id, currency_id) does the job. Faster and cleaner that way:
SELECT p.partner_id, pa.currency_id, sum(amount) AS sum_amount
FROM player_account pa
JOIN player p ON p.id = pa.player_id
WHERE p.partner_id = 819
AND pa.currency_id IN (1,2,3) -- may be redundant if there are not other
GROUP BY 1, 2;
If you need 1 row per partner_id, you are actually looking for "cross-tabulation" or a "pivot table". In Postgres use crosstab() from the additional module tablefunc , which is very fast. (Also available for the outdated version 8.4):
SELECT * FROM crosstab(
'SELECT p.partner_id, pa.currency_id, sum(amount)
FROM player_account pa
JOIN player p ON p.id = pa.player_id
WHERE p.partner_id = 819
AND pa.currency_id IN (1,2,3)
GROUP BY 1, 2
ORDER BY 1, 2'
,VALUES (1), (2), (3)'
) AS t (partner_id int, "curUsdAmount" numeric
, "curRubAmount" numeric
, "curUahAmount" numeric); -- guessing data types
Adapt to your actual data types.
Detailed explanation:
PostgreSQL Crosstab Query