Mind numbing SQL madness - mysql

Question

This query runs on an invoices table to help me decide who I need to pay
Here's the base table:
The users table
+---------+--------+
| user_id | name |
+---------+--------+
| 1 | Peter |
| 2 | Lois |
| 3 | Stewie |
+---------+--------+
The invoices table:
+------------+---------+----------+--------+---------------+---------+
| invoice_id | user_id | currency | amount | description | is_paid |
+------------+---------+----------+--------+---------------+---------+
| 1 | 1 | usd | 140 | Cow hoof | 0 |
| 2 | 1 | usd | 45 | Cow tail | 0 |
| 3 | 1 | gbp | 1 | Cow nostril | 0 |
| 4 | 2 | gbp | 1500 | Cow nose hair | 0 |
| 5 | 2 | cad | 1 | eyelash | 1 |
+------------+---------+----------+--------+---------------+---------+
I want a resulting table that looks like this:
+---------+-------+----------+-------------+
| user_id | name | currency | SUM(amount) |
+---------+-------+----------+-------------+
| 1 | Peter | usd | 185 |
| 2 | Lois | gbp | 1500 |
+---------+-------+----------+-------------+
The conditions are:
Only consider invoices that have not been paid, so where is_paid = 0
Group them by user_id, by currency
If the SUM(amount) < $100 for the user_id, currency pair then don't bother showing the result, since we don't pay invoices that are less than $100 (or equivalent, based on a fixed exchange rate).
Here's what I've got so far (not working -- which I guess is because I'm filtering by a GROUP'ed parameter):
SELECT
users.user_id, users.name,
invoices.currency, SUM(invoices.amount)
FROM
mydb.users,
mydb.invoices
WHERE
users.user_id = invoices.user_id AND
invoices.is_paid != true AND
SUM(invoices.amount) >=
CASE
WHEN invoices.currency = 'usd' THEN 100
WHEN invoices.currency = 'gbp' THEN 155
WHEN invoices.currency = 'cad' THEN 117
END
GROUP BY
invoices.currency, users.user_id
ORDER BY
users.name, invoices.currency;
Help?

Answer 1

You can't use SUM in a WHERE. Use HAVING instead.

Answer 2

Use HAVING clause instead of SUM in WHERE condition
Try this:
SELECT u.user_id, u.name, i.currency, SUM(i.amount) invoiceAmount
FROM mydb.users u
INNER JOIN mydb.invoices i ON u.user_id = i.user_id
WHERE i.is_paid = 0
GROUP BY u.user_id, i.currency
HAVING SUM(i.amount) >= (CASE i.currency WHEN 'usd' THEN 100 WHEN 'gbp' THEN 155 WHEN 'cad' THEN 117 END)
ORDER BY u.name, i.currency;

Answer 3

Try something like this:
SELECT
user_id, name, currency, sum(amount) due
FROM
invoice i
JOIN users u ON i.user_id=u.user_id
WHERE
is_paid = 0 AND
GROUP BY user_id, currency
having due >= 100
do you store exchange rates? Multiply rates with amount to get actual amount with respect to base currency.
sum(amount*ex_rate) due