怎麽两个SQL语句的结果会不一样呢？

Question

--语句一

SELECT str(sum(convert(dec(17,2),sysfiles.size)) / 128,10,2) AS Total_MB,
SUM(( database_files.size - FILEPROPERTY(database_files.name, 'SpaceUsed') )) / 128.0 AS free_space_mb
FROM    dbo.sysfiles as sysfiles,sys.database_files as database_files WHERE sysfiles.[groupid]  <>0
AND database_files.[type] <> 1;

--语句二

SELECT str(sum(convert(dec(17,2),sysfiles.size)) / 128,10,2) FROM   dbo.sysfiles WHERE sysfiles.[groupid]  <>0

想了半天未想明白

Answer 1

Result of 'select * from dbo.sysfiles where groupid <> 0'?

Answer 2

加上inner join之后好了

SELECT str(sum(convert(dec(17,2),sysfiles.size)) / 128,10,2) AS Total_MB,
SUM(( database_files.size - FILEPROPERTY(database_files.name, 'SpaceUsed') )) / 128.0 AS free_space_mb
FROM    dbo.sysfiles as sysfiles INNER JOIN sys.database_files as database_files ON sysfiles.[fileid]=database_files.[file_id] WHERE sysfiles.[groupid]  <>0
AND database_files.[type] <> 1;