Do indexes work with “IN” clause

Question

If I have a query like:

Select EmployeeId 
From Employee 
Where EmployeeTypeId IN (1,2,3)

and I have an index on the EmployeeTypeId

Answer 1

@Mike: Thanks for the detailed analysis. There are definately some interesting points you make there. The example I posted is somewhat trivial but the basis of the question came from using NHibernate.

With NHibernate, you can write a clause like this:

int[] employeeIds = new int[]{1, 5, 23463, 32523};
NHibernateSession.CreateCriteria(typeof(Employee))
.Add(Restrictions.InG("EmployeeId",employeeIds))

NHibernate then generates a query which looks like

select * from employee where employeeid in (1, 5, 23463, 32523)

So as you and others have pointed out, it looks like there are going to be times where an index will be used or a table scan will happen, but you can't really determine that until runtime.