inner join

内连接是inner join ，只返回两个表匹配的数据行。

select a.*,b.* from a inner join b on a.id = b.aid;
--等价于
select a.*,b.* from a  join b on a.id = b.aid;

left join 和right join

左外连接和右外连接

主表数据全部返回，连接表有匹配则返回，无匹配返回空值

select a.*,b.* from a left join b on a.id = b.aid;select a.*,b.* from a right join b on a.id = b.aid;

b on a.id = b.aid;