SQL查询语句

共有12个表，每个表对应一个月的数据，表的属性有machine_id（用户标识）,domain_id（网站标识）,现在要查询
1 全年中各用户访问的网站数：select machine_id,count(distinct domain_id) from T1,T2,T3,……,T12 group by machine_id
2 12个月总的网站数：select count(distinct domain_id)from T1,T2,……,T12
这两个查询语句好像不正确，后面是否要加where之类的语句，光是from好像不能连接12个表吧，应该怎么编写SQL语句，请指点，谢谢~

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union all

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1不是用连接是用UNION ALL
2也是用UNION ALL

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SQL code:

select machine_id,count(distinct domain_id) from
(select * from T1
union all
select * from T2
union all
……
select * from T12
)t
group by machine_id

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SQL code:
where t1.machine_id=t2.machine_id
and
.....

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SQL code:

select count(distinct domain_id) from
(select * from T1
union all
select * from T2
union all
……
select * from T12
)t

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理解错误？