-- try next:
select
(select idm from Tblmain where maincode = a.maincode) idm
,a.maincode
,(select mainnam from Tblmain where maincode = a.maincode) mainnam
,subname
from Tblsub
select
(select idm from Tblmain where maincode = a.maincode) idm
,a.maincode
,(select mainnam from Tblmain where maincode = a.maincode) mainnam
,subname
from Tblsub
ids maincode subname
1 001 猫
2 002 狗 --->这条记录的maincode是002吗？
如果是的话就没办法关联了