sql关键查询

情境：保留表A数据，且A表与B表是一对多关系

SELECT tuf.Id,tuf.FileName,tuf.type,tuf.url,tum.MachineId,tum.IsDownland,tum.IsFlag

FROM t_UpgradeFile tuf

LEFT join (SELECT t.* from  t_Upgrade_Machine t where t.MachineId='' ) tum on tuf.Id=tum.UpgradeFileId