Trying to use INNER JOIN and GROUP BY SQL with SUM Function, Not Working

Question

I am not getting my head around this, and wondered if anyone may be able to help me with this.

I have 2 Tables called RES_DATA and INV_DATA

RES_DATA Contains my Customer as below

CUSTOMER ID | NAME

1, Robert
2, John
3, Peter

INV_DATA Contains their INVOICES as Below

INVOICE ID | CUSTOMER ID | AMOUNT

100, 1, £49.95
200, 1, £105.95
300, 2, £400.00
400, 3, £150.00
500, 1, £25.00

I am Trying to write a SELECT STATEMENT Which will give me the results as Below.

CUSTOMER ID | NAME | TOTAL AMOUNT

1, Robert, £180.90
2, John, £400.00
3, Peter, £150.00

I think I need 2 INNER JOINS Somehow to Add the tables and SUM Values of the INVOICES Table GROUPED BY the Customer Table but honestly think I am missing something. Can't even get close to the Results I need.

Answer 1

This should work.

SELECT a.[CUSTOMER ID], a.[NAME], SUM(b.[AMOUNT]) AS [TOTAL AMOUNT]
FROM RES_DATA a INNER JOIN INV_DATA b
ON a.[CUSTOMER ID]=b.[CUSTOMER ID]
GROUP BY a.[CUSTOMER ID], a.[NAME]

I tested it with SQL Fiddle against SQL Server 2008: http://sqlfiddle.com/#!3/1cad5/1

Basically what's happening here is that, because of the join, you are getting the same row on the "left" (i.e. from the RES_DATA table) for every row on the "right" (i.e. the INV_DATA table) that has the same [CUSTOMER ID] value. When you group by just the columns on the left side, and then do a sum of just the [AMOUNT] column from the right side, it keeps the one row intact from the left side, and sums up the matching values from the right side.