【Hive-基础】HiveSql 连接查询【inner join、left join、right join、full join、union all、union】

1）inner join

1、连接 2 个表，取出公共部分

2、必须重命名

3、on后连接条件键值唯一

4、连接前注意去重，提高效率

5、inner可省略

select * from user_list_1 as a 
inner join user_list_2 as b
on a.user_id=b.user_id;

三表连接

select a.user_name 
from
(select distinct user_name from data1)as a 
inner join (select distinct user_name from data2) as b
on a.user_name=b.user_name
inner join (select distinct user_name from data3) as c 
on b.user_name=c.user_name;

2）left join、right join

left join：以左表为全集，返回能匹配上的匹配结果，没匹配上的显示 NULL；

right join：以右表为全集，返回能匹配上的匹配结果，没匹配上的显示 NULL；

应用：取出表1中存在，表2中不存在的项

select *
from user_list_1 a
left join user_list_2 b
on a.user_id=b.user_id;

3）full join

并列将2个表的信息展示

select *
from user_list_1 a
full join user_list_2 b
on a.user_id=b.user_id;

4）union all、union

增加行

1、字段名称必须一致

2、字段顺序必须一致

3、无连接条件

union all和union的区别： union 会去重且排序

• union all

select a.user_id,a.user_name
from user_list_1 a
union all
select b.user_id,b.user_name
from user_list_2 b;

• union

select a.user_id,a.user_name
from user_list_1 a
union
select b.user_id,b.user_name
from user_list_2 b;