SQL Server: how to use alias in update statement?

Question

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?

Answer 1

SQL Server supports updates using joins.
This means you can write your query like this:

UPDATE s
SET Value = d.NumOfRows
FROM statisticsTable s
INNER JOIN
(
     SELECT UserId, COUNT(*) As NumOfRows
     FROM OtherTable
     GROUP BY UserId
) d ON s.UserId = d.UserId
WHERE id in (1,2,3) 

Answer 2

Try this:

UPDATE s
   SET Value = x.cnt
from statisticsTable s
 outer apply (select count(*) as cnt
                FROM OtherTable o
                WHERE o.UserId = s.UserId ) x
WHERE s.id in (1,2,3)