在db_id（）中傳遞變量

我想知道爲什麼變量在「if語句」內的子查詢中傳遞時被區別對待，但在獨立執行時工作正常。在db_id（）中傳遞變量

成功的一個

select db_id((Select name from sys.databases where database_id > 4))

結果：

DBSIZE 
-------- 
5

當它返回null：

Declare @dbcount int; 

set @dbcount = (Select count(*) from sys.databases where database_id > 4); 

If @dbcount = 1 

Begin 

select usedspace as 'DBSize' 
from  
( 
SELECT (s.size * CONVERT(float,8))/1024 AS [Size], 
(CAST(CASE s.type WHEN 2 THEN 0 ELSE CAST(FILEPROPERTY(s.name, 'SpaceUsed') AS float)* CONVERT(float,8) END AS float))/1024 AS [UsedSpace], 
s.file_id AS [ID] 
FROM 
sys.filegroups AS g 
INNER JOIN sys.master_files AS s ON ((s.type = 2 or s.type = 0) and s.database_id = db_id((Select name from sys.databases where database_id > 4)) and (s.drop_lsn IS NULL)) AND (s.data_space_id=g.data_space_id) 
) DBFileSizeInfo 

End 

else 

Print 'The instance has more than 1 database'

結果：