左右连表

主表 left outer join 从表 on 主表.x = 从表.id

select * from info left outer join depart on info.depart_id = depart.id;

从表 right outer join 主表 on 主表.x = 从表.id

select info.id,info.name,info.email,depart.title from info right outer join depart on info.depart_id = depart.id;

内连接：

表 inner join 表 on 条件

到目前为止SQL执行顺序：
join
on
where
group by
having
order by
limit

union，上下连表，用的极少，自动去重

select id,title from depart
union
select id,name from info;