Any other way to solve this query?

Question

Table: Customer

customer_id  cust_name     city        grade       salesman_id
-----------  ------------  ----------  ----------  -----------
3002         Nick Rimando  New York    100         5001
3005         Graham Zusi   California  200         5002
3001         Brad Guzan    London                  5005
3004         Fabian Johns  Paris       300         5006
3007         Brad Davis    New York    200         5001
3009         Geoff Camero  Berlin      100         5003
3008         Julian Green  London      300         5002
3003         Jozy Altidor  Moncow      200         5007

Table: Order

ord_no      purch_amt   ord_date    customer_id  salesman_id
----------  ----------  ----------  -----------  -----------
70001       150.5       2012-10-05  3005         5002
70009       270.65      2012-09-10  3001         5005
70002       65.26       2012-10-05  3002         5001
70004       110.5       2012-08-17  3009         5003
70007       948.5       2012-09-10  3005         5002
70005       2400.6      2012-07-27  3007         5001
70008       5760        2012-09-10  3002         5001
70010       1983.43     2012-10-10  3004         5006
70003       2480.4      2012-10-10  3009         5003
70012       250.45      2012-06-27  3008         5002
70011       75.29       2012-08-17  3003         5007
70013       3045.6      2012-04-25  3002         5001

I need a query that produces the name and number of each customer with more than one current order.

My try:-

SELECT customer_id,  cust_name
FROM customer a
WHERE (SELECT COUNT (*)
       FROM orders b
       WHERE a.customer_id = b.customer_id) > 1

This gives desired output. But, is there any alternate way which would give desired output and perform better than this?Additionally,how could I know which one will do better?

Answer 1

I'd try this:

SELECT customer_id,  cust_name
FROM customer a
WHERE customer_id IN (SELECT customer_id
                      FROM orders b
                      GROUP BY customer_id
                      HAVING COUNT(*) > 1)

The sub-query simply returns the customer_id's that exists more one once in the orders table.

Or this, similar to the above, but a JOIN instead of IN:

SELECT a.customer_id, a.cust_name
FROM customer a
JOIN (SELECT customer_id
      FROM orders b
      GROUP BY customer_id
      HAVING COUNT(*) > 1) b ON a.customer_id = b.customer_id

Answer 2

You could have the query filtering customers with multiple orders as an inner query, and join on it:

SELECT c.customer_id, c.cust_name
FROM   customer c
JOIN   (SELECT   customer_id
        FROM     orders
        GROUP BY customer_id
        HAVING   COUNT(*) > 1) o ON c.customer_id = o.customer_id

Answer 3

No real need for an IN with a subquery.
This can be done via a join.

SELECT o.customer_id, c.cust_name, count(*) as total
FROM orders o 
LEFT JOIN customer c ON (o.customer_id = c.customer_id)
GROUP BY o.customer_id, c.cust_name
HAVING count(*) > 1;

As for comparing which query might perform better? Check out this old post about obtaining an query execution plan.

But if you need the "current" orders, you probably want to add a where clause to only select the orders of a recent date.

Answer 4

use join and Group by:

SELECT customer_id,  cust_name
FROM customer a join order a.customer_id = b.customer_id
Group by customer_id,  cust_name

The join only returns customer which have records in order table. Group by will prevent you for getting multiple values if there are more than one order.