Question

I got 2 tables like Customers and Orders, in table Customers I got columns id, name, in table Orders I got columns id, customer_id, order_date.

Now I need to make one select that will return me each Customer’s id, name and the last order_date.

I tried to make like this:

select
    Customers.id,
    Customers.name,
    (select Orders.order_date from Orders where Orders.customer_id = Customer.id order by order_date desc) as last_order_date
from
    Customers

But it get the wrong index and takes forever to execute.
Whats the best way to make this select in PostgreSQL?

Thanks in advanced.

Answer 1

If not restricting by customer_id, then the query will end up having to scan the entire orders table.

SELECT c.id
      ,c.name
      ,MAX(o.order_date) AS last_order_date
FROM Customers c
LEFT OUTER JOIN Orders o ON (o.customer_id = c.id)
GROUP BY c.id, c.name