多表查询：
连接查询：
内连接 inner join
select *
from table A,table B
where tableA.id = tableB .table_id
select *
from tableB inner join tableA
on tableB.tableA_id = tableB.id
外连接 left join right join
select *
from tableB left join tableA
on tableB.tableA_id = tableB.id
