MySQL - Joining two tables without duplicates?

Question

I have two tables that I am trying to join. One contains a list of customers, the other is a list of orders. I am trying to formulate a query that will allow me to select all of the customers listed in the table customers who have at least one order in the table orders. However, I do not want to get duplicates for those customers who have multiple orders. Any suggestions how I can accomplish this?

I know this is probably a common issue, however I have no idea what this type of query would be called so that I could search for an answer. Any suggestions would be greatly appreciated. Thanks.

Answer 1

It's much simpler than you may think:

select distinct(customer_id) from orders;

Edit: If you actually want to get the full info on the customer,

select * from customers where customer_id in (select distinct(customer_id) from orders);

Answer 2

Use:

SELECT c.*
  FROM CUSTOMERS c
 WHERE EXISTS (SELECT NULL
                 FROM ORDERS o
                WHERE o.custeromid = c.id)

The IN clause is an alternative, but EXISTS works better for duplicates because it returns true on the first duplicate so it doesn't process the entire table.