SQL Server: how to use alias in update statement?

I'm wondering about the following query :

   UPDATE statisticsTable
   SET Value = (select count(*) 
                FROM OtherTable o
                WHERE o.UserId = UserId ) <-- this is the part that concerns me
   WHERE id in (1,2,3)

How does SQL Server know that the second "UserId" field comes from statisticsTable and not from OtherTable ? Why can't I give an alias to statisticstable like 'stat' to clarify where I want to get that UserId ? Or is there a way?

SQL Server: how to use alias in update statement?

2 Answers