INNER JOIN：获取两个表中字段匹配关系的记录
select xxx from table1 t1 inner join table2 t2 on t1.xx=t2.xx;
inner join可以省略，等同于：
select xxx from table1 t1,table2 t2 where t1.xx=t2.xx;
LEFT JOIN：获取左表所有记录，即使右表没有对应匹配的记录
select xxx from table1 t1 left join table2 t2 on t1.xx=t2.xx;
RIGHT JOIN：获取右表所有记录，即使左表没有对应匹配的记录
select xxx from table1 t1 right join table2 t2 on t1.xx=t2.xx;