1.关于hive中的各种join
Hive中有许多的Join操作,例如:LEFT、RIGHT和FULL OUTER JOIN,INNER JOIN,LEFT SEMI JOIN等;
1.1.准备两组数据:
a.txt b.txt 1,a 2,bb 2,b 3,cc 3,c 7,yy 4,d 9,pp 7,y 8,u
1.2.在Hive中建表:
create table a(id int,name string) row format delimited fields terminated by ',';
create table b(id int,name string) row format delimited fields terminated by ',';
1.3.在Hive表中导入数据:
load data local inpath '/root/a.txt' into table a;
load data local inpath '/root/b.txt' into table b;
1.4.inner join内连接:select * from a inner join b on a.id=b.id;
相当于是求交集
1.5.left join左连接:select * from a left join b on a.id=b.id;(左边的表的记录都打印出来)
1.6.right join右连接:select * from a right join b on a.id=b.id;(右边的表的记录都打印出来)
1.7.full outer join右连接:select * from a full outer join b on a.id=b.id;
两边的记录都会出来,如果没有的就填NULL
1.7.left semi outer join连接:select * from a left semi join b on a.id = b.id;
SEMI在Hive中用的很广泛,用于实现Exist IN 子查询的。LEFT SEMI JOIN相当于Inner Join中取一半
----LEFT SEMI JOIN :Hive 当前没有实现 IN/EXISTS 子查询,可以用 LEFT SEMI JOIN 重写子查询语句
例如:重写以下子查询为LEFT SEMI JOIN
SELECT a.key, a.value FROM a WHERE a.key exist in (SELECT b.key FROM B);
可以被重写为:SELECT a.key, a.value FROM a LEFT SEMI JOIN b on (a.key = b.key);
查询与“刘晨”在同一个系学习的学生:
hive> select s1.Sname from student s1 left semi join student s2 on s1.Sdept=s2.Sdept and s2.Sname='刘晨';