SQL SUM, COUNT for only unique id - sql

Question

I want to calculate sum and count for only unique ids.
SELECT COUNT(orders.id), SUM(orders.total), SUM(orders.shipping) FROM "orders"
INNER JOIN "designer_orders" ON "designer_orders"."order_id" = "orders"."id"
WHERE (designer_orders.state = 'pending' OR
designer_orders.state = 'dispatched' OR
designer_orders.state = 'completed')
Do this only for unique orders ids.
Add orders.total only if orders.id is unique. Same goes for shipping.
Avoid adding duplicates.
For example, orders table inner joined designer_orders table:
OrderId Total Some designer order column
1 1000 2
1 1000 3
1 1000 5
2 100 7
3 133 8
4 1000 10
4 1000 20
In this case:
count of orders should be 4.
total of orders should be 2233.
Schema:
One order has many designer orders.
One designer order has only one order.

Answer 1

Try it this way
SELECT COUNT(o.id) no_of_orders,
SUM(o.total) total,
SUM(o.shipping) shipping
FROM orders o JOIN
(
SELECT DISTINCT order_id
FROM designer_orders
WHERE state IN('pending', 'dispatched', 'completed')
) d
ON o.id = d.order_id
Here is SQLFiddle demo

Answer 2

Since you are only interested whether any row with qualifying status exists in the table designer_orders, the most obvious query style would be an EXISTS semi-join. Typically fastest with potentially many duplicate rows in n-table:
SELECT COUNT(o.id) AS no_of_orders
,SUM(o.total) AS total
,SUM(o.shipping) AS shipping
FROM orders o
WHERE EXISTS (
SELECT 1
FROM designer_orders d
WHERE d.state = ANY('{pending, dispatched, completed}')
AND d.order_id = o.id
);
-> SQLfiddle demo
For fast SELECT queries with bigger tables (and at some cost for write performance), you would have a partial index like:
CREATE INDEX designer_orders_order_id_idx ON designer_orders (order_id)
WHERE state = ANY('{pending, dispatched, completed}');
The index condition must match the WHERE condition of the query to talk the query planner into actually using the index.
A partial index is particularly attractive if there are many rows with a status that does not qualify. Else, an index without condition might be the better choice overall.