Where IN a Comma delimited string

Create some split string function and convert the comma separated values to rows then you can use the converted rows IN clause

DECLARE @List VARCHAR(max)

SELECT @List = COALESCE(@List + ',', '') +StaffCode
FROM   tblStaffs

SELECT UserName
FROM   #temptable
WHERE  #temptable.StaffCode IN (SELECT split_values
                                FROM   dbo.Splitstring_function(@list)) 

Check here for various Split String function

If you dont want to create functions then you can also directly use the code instead of creating a new function(M.Ali's answer).

Another way of doing it is using dynamic query.

Declare @List varchar(max), @sql nvarchar(max)

Select @List = coalesce(@List + ',','') + '''' + StaffCode + ''''
From tblStaffs

set @sql = '
Select UserName
From #temptable
Where #temptable.StaffCode IN ('+ @List + ')'

--print @sql
exec (@sql)

To debug the dynamic query always print the dynamic sql before executing.

Because the Variable has a string which IN operator reads as 'AAA'',''ABB'',''BBB' and it treats it as a single value.

In your query you should really use the query itself in the IN operator something like....

Select UserName
From #temptable
Where #temptable.StaffCode IN (SELECT StaffCode From tblStaffs)

Anyway if there is a need to use variable and then read values inside the IN operator from that variable you can do something like this....

DECLARE @List VARCHAR(1000);

Select @List = coalesce(@List + ',','') + StaffCode 
From tblStaffs


SELECT *
From #temptable
Where #temptable.StaffCode IN (
     SELECT t.c.value('.', 'VARCHAR(1000)')
           FROM (
                 SELECT x = CAST('<t>' + 
                  REPLACE(@List , ',', '</t><t>') + '</t>' AS XML)
                ) a
     CROSS APPLY x.nodes('/t') t(c))