Multiple Tables, Inserts, Identity Columns And Database Integrity

Hi all,

I am writing a portion of an app that is of intensely high online eCommerce usage. I have a question about identity columns and locking or not.

What I am doing is, I have two tables (normalized), one is OrderDemographics(firstname,lastname,ccum,etc) the other is OrderItems. I have the primary key of OrderDemographics as a column called 'ID' (an Identity Integer that is incrementing). In the OrderItems table, the 'OrderID' column is a foreign key to the OrderDemographics Primary Key column 'ID'.

What I have previously done is to insert the demographics into OrderDemographics, then do a 'select top 1 ID from OrderDemographics order by ID DESC' to get that last ID, since you can't tell what it is until you add another row....

The problem is, there's up to 20,000 users/sessions at once and there is a possiblity that in the fraction of a second it takes to select back that ID integer and use it for the initial OrderItems row, some other user might have clicked 'order' a fraction of a second after the first user and created another row in OrderDemographics, thus incrementing the ID column and throwing all the items that Customer #1 orders into Customer #2's order....

How do I lock a SQL table or lock the Application in .NET to handle this problem and keep it from occurring?

Thanks, appreciate it.