Question

What I want to do is have a table structure like this:

TypeID   int       not null (foreign key)
ItemID   int       (computed value +1 for each value with same TypeId) 
Data     string

e.g.

TypeId  ItemId  Data 
1       1       "some data"
1       2       "some data"
2       1       "some data"
3       1       "some data"
3       2       "some data"

If I have computed columns as a key, am I going to have issues with concurrency as opposed to identity columns?

Answer 1

The ItemId column seems to be the result of a ROW_NUMBER OVER(PARTITION BY TypeID) (see the docs) — is that how you’re computing it? As such it’s not really suitable as part of a key due to concurrency issues (two transactions inserting rows with the same TypeID, as you mention). But what does that have to do with “how to increment a subset”? What subset of what? And what’s your question? Why not just make something else the key instead (typically an artificial auto-increment column)?