Auto-increment the primary key without setting the identity

Question

I am using the following syntax to insert new records (I don't want to have duplicate keys):

insert into tbl(key) values select max(key)+1 from tbl

Someone says it will have concurrency problem.
Is that right?

SELECT -> LOCK table -> INSERT

or

LOCK table -> SELECT -> INSERT

Which one of the above is correct?

Answer 1

If this happens inside a transaction you will be fine.