p2p.wrox.com Forums - Performing Calculations with User Define Function

I know the problem here is me and so I need to know how to get this right, please!

I created these two tables to illustrate what I am trying to accomplished.

I am trying to calculate the total amount from the tblSupportCharges table using the CustomerID as an Input Parameter.
The problem is my calculations are dead wrong.

Here are the two tables:

Code:

CREATE TABLE tblTechSupport (

CustomerId        INT IDENTITY(1,1)NOT NULL,

CONSTRAINT PK_tblTechSupport PRIMARY KEY (CustomerId),

CompanyName VARCHAR(50)  NOT NULL,        

CompanyLoc  VARCHAR(50)  NOT NULL,        

CallDate        DATE         NOT NULL,                                                                   

RepName                CHAR(50)     NOT NULL

);



CREATE TABLE tblSupportCharges(

CustomerId     INT          NOT NULL,

CONSTRAINT FK_tblSupCharges FOREIGN KEY (CustomerId)

REFERENCES tblTechSupport (CustomerId),                 

[SupportHours] DECIMAL(9,2) NULL,

CostPerHour    MONEY        NULL,        

MilesTraveled  DECIMAL(9,1) NULL,

CostPer_Mile   MONEY        NULL, 

PartsCosts     MONEY        NULL,

LaborCharges   MONEY            NULL

);



INSERT INTO tblSupportCharges(CustomerId,SupportHours,CostPerHour,MilesTraveled,CostPer_Mile)

VALUES(1,5,1,1,1);

Using the values I inserted, I created this function to get the total from the tblSupportCharges table but the results are not accurate: Here is the function: Note I made the numbers simple just for troubleshooting.

Code:

Create Function dbo.CalulateTotal (@CustomerId INT) 

RETURNS MONEY 

As 

BEGIN 

DECLARE @GetTotalCost MONEY         

SELECT  @GetTotalCost = (Sum(SupportHours * CostPerHour) + (Sum(MilesTraveled * CostPer_Mile)))

FROM tblSupportCharges                 

Return(@GetTotalCost) 

End 

Go 

Select dbo.CalulateTotal(1)

Thanks everyone!