Functional key and concurrency

Question

Context: web application, SQL Server / SQL Azure through EF Code first, asp.net mvc 3, windows azure.

I have a TPH table company which contains both customer companies and provider companies. The table contains a column type , it's the table discriminator: c for customers and p for providers.

The primary key of the table is an auto incremented integer id , managed by the database.

Now I would like to have a functional compouned key, ( short_id, type ), where short_id is also an autoincremented integer but I don't know how and when generate short_id values to avoid concurrency issues.

How can I prevent two different customer companies to get the same short_id if one is entered in the database in new york at the very same time the other one is entered in paris.

id name       type  short_id
1  company_a  c    1
2  company_b  c    2
3  company_c  p    1
4  company_d  c    3
5  company_e  p    2
6  company_f  c    4

I know it's unlikely since the example is dumb but the question is technical.

Thank you for reading

Answer 1

You might try this, though it could get slow if the table is large:

INSERT INTO company (name, type, short_id)
SELECT 'mycompany', 'c', ISNULL(MAX(short_id),0) + 1
FROM company
WHERE type = 'c'