hive的join查询
语法
join_table:
table_reference [INNER] JOIN table_factor [join_condition]
| table_reference {LEFT|RIGHT|FULL} [OUTER] JOIN table_reference join_condition
| table_reference LEFT SEMI JOIN table_reference join_condition
| table_reference CROSS JOIN table_reference [join_condition] (as of Hive 0.10)
table_reference:
table_factor
| join_table
table_factor:
tbl_name [alias]
| table_subquery alias
| ( table_references )
join_condition:
ON expression
// join查询不支持where，改用on来替代
实例：
select count(distinct a.id) from table_1 a join table_2 b on (a.id=b.id) ;
left join/right join
左查询就是不管能不能匹配到右边的表，都会输出左面的表，未匹配到的项目为空
| name | age |
|---|---|
| wang | 13 |
| zhao | 14 |
| name | score |
|---|---|
| wang | 145 |
| li | 44 |
select a.name, b.score from t1 a join t2 b on (a.aid=b.aid);
的结果是：
| name | score |
|---|---|
| wang | 145 |
| zhao | NULL |
注意点
- 注意，下面这些操作是非法的：
CREATE TABLE a (k1 string, v1 string);
CREATE TABLE b (k2 string, v2 string);
SELECT k1, v1, k2, v2
FROM a JOIN b ON k1 = k2;
// 选择每个属性的时候需要制定是哪个表，否则会认为有歧义
- 只支持相等查询的join操作
SELECT a.* FROM a JOIN b ON (a.id <> b.id)
// 非法操作
SELECT a.* FROM a LEFT OUTER JOIN b ON (a.id <> b.id)
// 合法，此处是left outer join
- 可以支持多个表的查询
SELECT a.val, b.val, c.val FROM a JOIN b ON (a.key = b.key1) JOIN c ON (c.key = b.key2)