内连接:[inner|cross] join
(不带inner或cross时也值内连接)
外连接:
左外连接:left [outer] join
右外连接:right [outer] join
错误语法:
outer join
full outer join
mysql不支持全连接
全连接实现方法
select a.*,b.* from a left join b on a.id=b.id
union
select a.*,b.* from a right join b on a.id=b.id and a.id is null