Operation on each row from select query - sql

Question

How execute additional query (UPDATE) on each row from SELECT?
I have to get amount from each row from select and send it to user's balance table.
Example:
status 0 - open
status 1 - processed
status 2 - closed
My select statement:
select id, user_id, sell_amount, sell_currency_id
from (select id, user_id, sell_amount, sell_currency_id,
sum(sell_amount)
over (order by buy_amount/sell_amount ASC, date_add ASC) as cumsell
from market t
where (status = 0 or status = 1) and type = 0
) t
where 0 <= cumsell and 7 > cumsell - sell_amount;
Select result from market table
id;user_id;amount;status
4;1;1.00000000;0
6;2;2.60000000;0
5;3;2.00000000;0
7;4;4.00000000;0
We get 7 amount and send it to user balance table.
id;user_id;amount;status
4;1;0.00000000;2 -- took 1, sum 1, status changed to 2
6;2;0.00000000;2 -- took 2.6, sum=3.6, status changed to 2
5;3;0.00000000;2 -- took 2, sum 5.6, status changed to 2
7;4;2.60000000;1 -- took 1.4, sum 7.0, status changed to 1 (because there left 2.6 to close)
User's balance table
user_id;balance
5;7 -- added 7 from previous operation
Postgres version 9.3

Answer 1

The general principle is to use UPDATE ... FROM over a subquery. Your example is too hard to turn into useful CREATE TABLE and SELECT statements, so I've made up a quick dummy dataset:
CREATE TABLE balances (user_id integer, balance numeric);
INSERT INTO balances (user_id, balance) VALUES (1,0), (2, 2.1), (3, 99);
CREATE TABLE transactions (user_id integer, amount numeric, applied boolean default 'f');
INSERT INTO transactions (user_id, amount) VALUES (1, 22), (1, 10), (2, -10), (4, 1000000);
If you wanted to apply the transactions to the balances you would do something like:
BEGIN;
LOCK TABLE balances IN EXCLUSIVE MODE;
LOCK TABLE transactions IN EXCLUSIVE MODE;
UPDATE balances SET balance = balance + t.amount
FROM (
SELECT t2.user_id, sum(t2.amount) AS amount
FROM transactions t2
GROUP BY t2.user_id
) t
WHERE balances.user_id = t.user_id;
UPDATE transactions
SET applied = true
FROM balances b
WHERE transactions.user_id = b.user_id;
The LOCK statements are important for correctness in the presence of concurrent inserts/updates.
The second UPDATE marks the transactions as applied; you might not need something like that in your design.