inner join和cross join的使用

inner join和cross join的使用和逗號是同樣的效果，其中的區別就是，inner join產生的是交叉鏈接，而cross join是產生內鏈接而已。

例：

select countryname,statename from country,state 等價於select countryname,statename from country cross join state；

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

 

並且，這兩個關鍵字和逗號均可以儘量多的鏈接不少的表：

select * from country cross join state cross join city;