access 支持三表或三表以上的 join 操作，但是要加括号，一层一层 join 下去
例子：
1、select * from xxas a left outer join yy as b on a.url=b.url
2、select * from (xxas a left outer join yy as b on a.url=b.url) left outer join zz as c on c.url=a.url
3、select * into all_ from ((((xx as a left outer join yy as b on a.url=b.url)
left outer join zz as c on c.url=a.url)
left outer join mm as d on d.url=a.url)
left outer join nn as e on e.url=a.url)