sql中將查詢到的結果生成一個虛擬表，再進行操做

舉例：select count(distinct A.questno) from (select questno,fastdfsurl,docfastdfsurl,pngfastdfsurl from   table1 where fastdfsurl like 'group%'  and docfastdfsurl like 'group%' and pngfastdfsurl like 'gr

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