Stored Procedure That Checks To See Whether A Value Exists And Only Enters Data If It Doesn't

Hi All,

I Have a nifty little stored procedure that takes data from an ASP form post
and inserts the data into two tables. However what I really need this to do
before the insert is check whether there is already a record that matches
some of the criteria, and if so returns an error "This Username already
exists" and only if there isn't a record that matches the criteria is the
record inserted.

my current stored procedure looks like this -

Code Snippet@siteid int,@companyname nvarchar(50),@address nvarchar(500),@phone nvarchar(50),@fax nvarchar(50),@email nvarchar(225),@url nvarchar(225),@companytype nvarchar(50),@billingcontact nvarchar(50),@name nvarchar(50),@AccountType nvarchar(50),@PASSWORD nvarchar(50),@AccountLive nvarchar(50),@EmployeeLevel nvarchar(50)ASDeclare @NewID INTINSERT INTO dbo.JBClient(JBCLSiteID, JBCLName, JBCLAddress, JBCLPhone, JBCLFax, JBCLEmail, JBCLCompanyType, JBCLURL, JBCLAccountType, JBCLAccountlive, JBCLBillingContact)VALUES (@siteid, @companyname, @address, @phone, @fax, @email, @companytype, @url, @AccountType, @AccountLive, @billingcontact)SELECT @NewID = SCOPE_IDENTITY()INSERT INTO dbo.JBEmployee(JBEClientID, JBESiteID, JBEName, JBELevel, JBEUsername, JBEPassword, JBEAddress, JBEPhone)VALUES (@NewID, @siteid, @name, @EmployeeLevel, @email, @PASSWORD, @address, @phone)
The values that i need to check against are -

@siteid
@email

in the table dbo.JBEmployee against columns JBESiteID & JBEUsername

What i would really like to do if a record exists is return the user to an
ASP page, which contains all of the variables previously enterred -,

@siteid int,
@companyname nvarchar(50),
@address nvarchar(500),
@phone nvarchar(50),
@fax nvarchar(50),
@email nvarchar(225),
@url nvarchar(225),
@companytype nvarchar(50),
@billingcontact nvarchar(50),
@name nvarchar(50),
@AccountType nvarchar(50),
@PASSWORD nvarchar(50),
@AccountLive nvarchar(50),
@EmployeeLevel nvarchar(50)

together with a message that says "This Username already exists"

Does anyone have any idea how to do this???

Many thanks