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I have two tables, orders and customers, and I’d like to find all customers where orders is greater than 0.

I currently use two queries:

SELECT * FROM customers 

and

SELECT count(id) 
FROM orders 
WHERE customer='CUSTOMER_VALUE' 
AND siteid='siteid'

I’d like to turn this into one query, so it finds all customers where they’ve placed one or more orders in the store.

I tried the following, but it doesn’t work:

SELECT c.*, 
       COUNT(o.customer) AS numOrders 
FROM customers c, 
     orders o 
WHERE o.siteid= 'calico' 
AND o.customer=c.email

(it only gives one result, which is customer ID 1).

I only need to find the orders value, and a few values from the customers table.

The customer field in orders and the id field in customers are the same value (ie order 1 was placed by customer 5, customer id 5 is “John”).

Is there any way to do this?

The current way works, but it would be greatly inefficient if there was to be a large amount of customers.

Answer 1

The reason your second query is returning only 1 row, is because oyu dont have a GROUP BY. Unlike many SQL databases, MySQL does allow you to mix non-aggregated fields with aggregated ones, even though its technically not valid sql, and the results are unpredictable.

Try

SELECT c.id, c.email, COUNT(o.customer) AS numOrders 
 FROM customers c 
  INNER JOIN orders o on (o.customer=c.email) 
  WHERE o.siteid= 'calico' 
 GROUP BY c.id, c.email