内连接与外连接

一、内连接

1、以下两个语句查询结果一样

select a.dept_name, a.id, b.prem_codes, c.scy_lvl
from T_DA_MGR_ORG  a
join  T_DA_ORG_SCY_REL b
on a.id = b.dept_id
join T_DA_SCY_LVL c
on b.scy_lvl = c.scy_lvl

select a.dept_name, a.id, b.prem_codes, c.scy_lvl
from T_DA_MGR_ORG a, T_DA_ORG_SCY_REL b, T_DA_SCY_LVL c
where a.id = b.dept_id
and b.scy_lvl = c.scy_lvl

2、单行子查询 where =、>、 >= 、= 、 <、<=、between and、and、or、in

select * from stuinfo t

      where t.classno in

          (select b.classno  from class b  where b.classname = '信息科学2班（18）'   );

3、多行子查询 ANY、ALL、some 

select * from stuinfo t where t.age>any(26,27,28); ---年龄只要大于当中子查询的最小值26岁即可
select * from stuinfo t where t.age>all(26,27,28); ---年龄必须大于子查询当中的最大值28岁才可以

4、为计算字段使用子查询

select cust_name,
cust_state,
(select count(*) from orders where orders.id = customers.cust_id) as orders
from customers

二、外连接

1、左外连接

select a.dept_name, a.id, b.prem_codes, c.scy_lvl
from T_DA_MGR_ORG a
left join T_DA_ORG_SCY_REL b
on a.id = b.dept_id
left join T_DA_SCY_LVL c
on b.scy_lvl = c.scy_lvl

select a.dept_name, a.id, b.prem_codes, c.scy_lvl
from T_DA_MGR_ORG a, T_DA_ORG_SCY_REL b, T_DA_SCY_LVL c
where a.id = b.dept_id(+)
and b.scy_lvl = c.scy_lvl(+)

2、右外连接

select a.dept_name, a.id, b.prem_codes, c.scy_lvl
from T_DA_MGR_ORG a
right join T_DA_ORG_SCY_REL b
on a.id = b.dept_id
right join T_DA_SCY_LVL c
on b.scy_lvl = c.scy_lvl

select a.dept_name, a.id, b.prem_codes, c.scy_lvl
from T_DA_MGR_ORG a, T_DA_ORG_SCY_REL b, T_DA_SCY_LVL c
where a.id(+) = b.dept_id
and b.scy_lvl (+)= c.scy_lvl

3、全外连接

select a.*, b.stuid, b.stuname
from 表1 a
full join 表2  b
on a.stuid = b.stuid;

 

标签：DA,SCY,scy,id,lvl,连接,select
来源： https://www.cnblogs.com/diandixingyun/p/11608723.html