inner join和cross join的使用

inner join和cross join的使用和逗号是同样的效果，其中的区别就是，inner join产生的是交叉链接，而cross join是产生内链接而已。

例：

select countryname,statename from country,state 等价于select countryname,statename from country cross join state；

select countryname,statename from country,state where state.cid = country.id 等价于 elect countryname,statename from country inner join state where state.cid = country.id;

 

并且，这两个关键字和逗号均可以尽量多的链接不少的表：

select * from country cross join state cross join city;