How can I use a string list from a select statement in a NOT IN clause

Question

I want to build a list of Lastnames to filter out from the results of the second select statement something like this (which doesn't do it)

DECLARE @ExcludedList VARCHAR(MAX)
SET @ExcludedList = (SELECT  TOP 20 Lastname
FROM dbo.Subscribers
where [Firstname] = 'Dave')

SELECT [Firstname], Count([Firstname]) as [Count] from Subscribers
WHERE [Lastname] not in (@ExcludedList) 

Answer 1

If you need to use a variable, then it has to be a table variable:

DECLARE @ExcludedList TABLE (Lastname VARCHAR(MAX))

INSERT INTO @ExcludedList
SELECT  TOP 20 Lastname
FROM dbo.Subscribers
WHERE [Firstname] = 'Dave'

SELECT [Firstname], Count([Firstname]) AS [Count] 
FROM Subscribers
WHERE [Lastname] NOT IN (SELECT Lastname FROM @ExcludedList)