Select sum and inner join - sql

Question

I have two tables
Bills: id amount reference
Transactions: id reference amount
The following SQL query
SELECT
*,
(SELECT SUM(amount)
FROM transactions
WHERE transactions.reference = bils.reference) AS paid
FROM bills
GROUP BY id HAVING paid<amount
was meant to some rows from table Bills, adding a column paid with the sum of amount of related transactions.
However, it only works when there is at least one transaction for each bill. Otherwise, no line for a transaction-less bill is returned.
Probably, that's because I should have done an inner join!
So I try the following:
SELECT
*,
(SELECT SUM(transactions.amount)
FROM transactions
INNER JOIN bills ON transactions.reference = bills.reference) AS paid
FROM bills
GROUP BY id
HAVING paid < amount
However, this returns the same value of paid for all rows! What am I doing wrong ?

Answer 1

Use a left join instead of a subquery:
select b.id, b.amount, b.paid, sum(t.amount) as transactionamount
from bills b
left join transactions t on t.reference = b.reference
group by b.id, b.amount, b.paid
having b.paid < b.amount
Edit:
To compare the sum of transactions to the amount, handle the null value that you get when there are no transactions:
having isnull(sum(t.amount), 0) < b.amount

Answer 2

You need a RIGHT JOIN to include all bill rows.
EDIT
So the final query will be
SELECT
*,
(SELECT SUM(transactions.amount)
FROM transactions
WHERE transactions.reference = bills.reference) AS paid
FROM bills
WHERE paid < amount

Answer 3

I knows this thread is old, but I came here today because I encountering the same problem.
Please see another post with same question:
Sum on a left join SQL
As the answer says, use GROUP BY on the left table. This way you get all the records out from left table, and it sums the corresponding rows from right table.
Try to use this:
SELECT
*,
SUM(transactions.sum)
FROM
bills
RIGHT JOIN
transactions
ON
bills.reference = transactions.reference
WHERE
transactions.sum > 0
GROUP BY
bills.id