Behaviour of the brackets in a Select clause

Question

Dear Readers,
I can't understand why I get 2 different results on running with a Bracket I get NULL and without a bracket I get the declared variable value which is Noname
Below is Query 1:
Declare @testvar char(20)
Set @testvar = 'noname'
Select @testvar= pub_name
FROM publishers
WHERE  pub_id= '999'
Select @testvar
Out put of this query is 'Noname'
BUT when I type the same query in the following manner I get Null-------Please note that the only difference between this query below is I used brackets and Select  in the Select@testvar statement
Declare @testvar char(20)
Set @testvar = 'noname'
Select @testvar=(Select pub_name
FROM publishers
WHERE  pub_id= '999')
Select @testvar
Thank-you

Answer 1

Yes, this is plain-table valued query:
Select au_id
From Authors
WHERE 1=0
But here
Select(Select au_id
FROM authors
WHERE 1=0)
you have a query on the form
   SELECT (scalar value)
The scalar value in this case is a subquery, a scalar subquery. A scalar subquery always returns a single row. If the set returned by the query is empty, the scalar return value is NULL. If the set includes more than one value, this is an error.
A different way to describe this is that the context is scalar, and this is why the subquery is scalar. All values in a SELECT list must be scalar with regards to the row returned. If you say
SELECT expr FROM tbl
And there are five rows in table, the query will return five rows. If expr is a nested query, this cannot lead to the query returning 3 or 33 rows. The query will return five rows, period.
There is also:
 Select au_id
 FROM  (Select au_id
        FROM authors
        WHERE 1=0) AS k
Here the context is tabular, and in this example you will get no rows back. With a diffrent WHERE clause you can get many rows back.
Erland Sommarskog, SQL Server MVP, [email protected]