请问高手
select u.name, o.name, decode(o.type#, 2, 'TABLE', 3, 'CLUSTER')
from sys.user$ u, sys.obj$ o, sys.tab$ t
where o.owner# = u.user#
and o.obj# = t.obj# (+)
and t.bobj# is null
and o.type# in (2,3)
and o.linkname is null
这条查询语句中的$是何意义,整个查询语句的功能是什么?