求sql指导

select xxx from table
left outer join(
select count(*) as emails
from（select .....
) as emails_table on true
那个as emails_table on true是有什么用的？
left outer join(
select count(*) as emails
from (select...
)as moo
) as emails_table on true
这个as moo又有什么用呢
求指导，谢谢