Help with a subquery

Question

Hi all,
I'm having nightmares with a query I just cant think. I hope some one can help me with this.
There are 4 tables
customers ->Cust_id (PK), cust_name, cust_lname, ...
Invoice -> Invoice_number (PK), cust_id (FK), invoice_date
Invoice_line -> Invoice_number(FK), (FK)Prod_code, line_price, line_units
Product -> Prod_code,.....etc
And I need to make a query that show me the customer name that spent more money.
The money is obtained as (line_price * line_units), but there are several Invoice_line rows in each Invoice
So Ive done this so far but I don't know how to extract the max value and linked it to the customer name so I can just show the customer name (cust_fname) and last name (cust_lname).
select i.cust_id, sum(line_price*line_units)
from Invoice_Line il join invoice i
on il.invoice_number=i.Invoice_Number
group by i.Cust_Id;

Answer 1

Hi,
Welcome to the forum!
So you can get the total amount for every customer, but now you need to narrow that down to just the top-spending customer.
One way is to number each customer according to how much they spent, then show only #1:
WITH     got_cust_amt     AS
     select        i.cust_id
     ,       sum (line_price * line_units)     AS cust_amt
     ,       RANK () OVER (ORDER BY  sum (line_price * line_units))
                              AS r_num
     from       Invoice_Line     il
     join       invoice      i  on      il.invoice_number  = i.Invoice_Number
     group by  i.Cust_Id
SELECT     c.cust_id
,     c.cust_name
,     c.cust_lname
,     a.cust_amt
FROM     customers     c
JOIN     got_gust_amt     a  ON     c.cust_id     = a.cust_id
WHERE     a.r_num     = 1
;I like the method above, because it can easily be modified in case you need the top 5 customers, or the highest spender each year. In the event of a tie, it shows all the customers who have a claim to being the highest spender.
Here's another variation on the same idea:
WITH     got_cust_amt     AS
     select        i.cust_id
     ,       sum (line_price * line_units)     AS cust_amt
     from       Invoice_Line     il
     join       invoice      i  on      il.invoice_number  = i.Invoice_Number
     group by  i.Cust_Id
SELECT     c.cust_id
,     c.cust_name
,     c.cust_lname
,     a.cust_amt
FROM     customers     c
JOIN     got_cust_amt     a  ON     c.cust_id     = a.cust_id
WHERE     c.cust_amt = (
               SELECT  MAX (cust_amt)
               FROM     got_cust_amt
;This 2nd way can't be easily modified to show the top 5.
Notice that the sub-query, got_cust_amt, is just what you wrote already (plus another column, in the first version).
Edited by: Frank Kulash on Nov 10, 2010 10:54 AM
Corrected mis-spelling.