Using a column in sql join without adding it to group by clause - sql

Question

My actual table structures are much more complex but following are two simplified table definitions:
Table invoice
CREATE TABLE invoice (
id integer NOT NULL,
create_datetime timestamp with time zone NOT NULL,
total numeric(22,10) NOT NULL
);
id create_datetime total
----------------------------
100 2014-05-08 1000
Table payment_invoice
CREATE TABLE payment_invoice (
invoice_id integer,
amount numeric(22,10)
);
invoice_id amount
-------------------
100 100
100 200
100 150
I want to select the data by joining above 2 tables and selected data should look like:-
month total_invoice_count outstanding_balance
05/2014 1 550
The query I am using:
select
to_char(date_trunc('month', i.create_datetime), 'MM/YYYY') as month,
count(i.id) as total_invoice_count,
(sum(i.total) - sum(pi.amount)) as outstanding_balance
from invoice i
join payment_invoice pi on i.id=pi.invoice_id
group by date_trunc('month', i.create_datetime)
order by date_trunc('month', i.create_datetime);
Above query is giving me incorrect results as sum(i.total) - sum(pi.amount) returns (1000 + 1000 + 1000) - (100 + 200 + 150) = 2550.
I want it to return (1000) - (100 + 200 + 150) = 550
And I cannot change it to i.total - sum(pi.amount), because then I am forced to add i.total column to group by clause and that I don't want to do.

Answer 1

You need a single row per invoice, so aggregate payment_invoice first - best before you join.
When the whole table is selected, it's typically fastest to aggregate first and join later:
SELECT to_char(date_trunc('month', i.create_datetime), 'MM/YYYY') AS month
, count(*) AS total_invoice_count
, (sum(i.total) - COALESCE(sum(pi.paid), 0)) AS outstanding_balance
FROM invoice i
LEFT JOIN (
SELECT invoice_id AS id, sum(amount) AS paid
FROM payment_invoice pi
GROUP BY 1
) pi USING (id)
GROUP BY date_trunc('month', i.create_datetime)
ORDER BY date_trunc('month', i.create_datetime);
LEFT JOIN is essential here. You do not want to loose invoices that have no corresponding rows in payment_invoice (yet), which would happen with a plain JOIN.
Accordingly, use COALESCE() for the sum of payments, which might be NULL.
SQL Fiddle with improved test case.

Answer 2

Do the aggregation in two steps. First aggregate to a single line per invoice, then to a single line per month:
select
to_char(date_trunc('month', t.create_datetime), 'MM/YYYY') as month,
count(*) as total_invoice_count,
(sum(t.total) - sum(t.amount)) as outstanding_balance
from (
select i.create_datetime, i.total, sum(pi.amount) amount
from invoice i
join payment_invoice pi on i.id=pi.invoice_id
group by i.id, i.total
) t
group by date_trunc('month', t.create_datetime)
order by date_trunc('month', t.create_datetime);

Answer 3

See sqlFiddle
SELECT TO_CHAR(invoice.create_datetime, 'MM/YYYY') as month,
COUNT(invoice.create_datetime) as total_invoice_count,
invoice.total - payments.sum_amount as outstanding_balance
FROM invoice
JOIN
(
SELECT invoice_id, SUM(amount) AS sum_amount
FROM payment_invoice
GROUP BY invoice_id
) payments
ON invoice.id = payments.invoice_id
GROUP BY TO_CHAR(invoice.create_datetime, 'MM/YYYY'),
invoice.total - payments.sum_amount