在做sql 操作的時候多表join 在所難免，下面主要介紹一下hive 中做join

LEFT JOIN，RIGHT JOIN， FULL OUTER JOIN ,inner join, left semi join

準備資料
1,a
2,b
3,c
4,d
7,y
8,u

2,bb
3,cc
7,yy
9,pp

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 ',';
匯入資料：

load data local inpath '/home/hadoop/a.txt' into table a;
load data local inpath '/home/hadoop/b.txt' into table b;
1. inner join
select * from a inner join b on a.id=b.id;
+-------+---------+-------+---------+--+
| a.id | a.name | b.id | b.name |
+-------+---------+-------+---------+--+
| 2 | b | 2 | bb |
| 3 | c | 3 | cc |
| 7 | y | 7 | yy |
+-------+---------+-------+---------+--+
就是求交集。

2. inner join
select * from a left join b on a.id=b.id;
+-------+---------+-------+---------+--+
| a.id | a.name | b.id | b.name |
+-------+---------+-------+---------+--+
| 1 | a | NULL | NULL |
| 2 | b | 2 | bb |
| 3 | c | 3 | cc |
| 4 | d | NULL | NULL |
| 7 | y | 7 | yy |
| 8 | u | NULL | NULL |
+-------+---------+-------+---------+--+
左邊沒有找到連線的置空。

3. right join
select * from a right join b on a.id=b.id;
4. full outer join
select * from a full outer join b on a.id=b.id;
+-------+---------+-------+---------+--+
| a.id | a.name | b.id | b.name |
+-------+---------+-------+---------+--+
| 1 | a | NULL | NULL |
| 2 | b | 2 | bb |
| 3 | c | 3 | cc |
| 4 | d | NULL | NULL |
| 7 | y | 7 | yy |
| 8 | u | NULL | NULL |
| NULL | NULL | 9 | pp |
+-------+---------+-------+---------+--+
兩邊資料都顯示。

5. left semi join
select * from a left semi join b on a.id = b.id;
+-------+---------+--+
| a.id | a.name |
+-------+---------+--+
| 2 | b |
| 3 | c |
| 7 | y |
+-------+---------+--+
只返回左邊一半，即a的東西，效率高一點。


 

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