SQL Server which query runs faster - sql

Question

UPDATE User
SET Name = (SELECT NameSpace.NameId
FROM NameSpace
WHERE NameSpace.Name = 'BlaBlaBla')
WHERE UserId = 1453
This is faster or
int Value = Select NameSpace.NameId from NameSpace
where NameSpace.Name = 'BlaBlaBla';
UPDATE User
SET Name = "+Value +"
WHERE UserId = 1453
and
Select
UserName,
UserAge,
(Select * from AdressesTable where Adresses.AdresID=User.AdresID)
from
UserTable
where
UserId='123'
OR
Select *
from AdressesTable, UserTable
where Adresses.AdresID = User.AdresID AND UserID = '123'

Answer 1

There are a variety of assumptions to be made in determining which is faster.
First, if you are concerned about speed, then you want indexes on users(userid) and namespace(name).
Second, the assignment query should look like this in SQL Server:
declare #Value int;
select #Value = NameSpace.NameId
from NameSpace
where NameSpace.Name = 'BlaBlaBla';
Your variable declarations and subqueries are not correct for SQL Server.
Finally, even with everything set up correctly, it is not possible to say which is faster. If I assume that there is only one matching record for UserId, then the single update is probably faster -- although perhaps by so little that it is not noticeable. It may not be faster. The update may cause some sort of lock to be taken on NameSpace that would not otherwise be taken. I would actually expect the two to be quite comparable in speed.
However, if many users have the same userid (which is unlikely given the name of the column), then you are doing updates on multiple rows. Storing the calculated result once and using that is probably better than running the subquery multiple times. Even so, with the right indexes, I would expect the difference in performance to be negligible.