Question

我有一个表，其中记录了同时订阅英语和北印度语频道的用户记录，我只希望那些订阅英语的用户

+--------+------+------+------+
| id | userid | Subscribedto |   
+--------+------+------+------+
|  1 |   1     |   English   |
|  2 |   2     |   English   |
|  3 |   1     |   Hindi     |
|  4 |   3     |   English   |
|  4 |   3     |   Hindi     |
|  5 |   4     |   English   |
+--------+------+------+------+

结果将是

+--------+------+------+------+
| id | userid | Subscribedto |   
+--------+------+------+------+
|  2 |   2     |   English   |
|  5 |   4     |   English   |
+--------+------+------+------+

Answer 1

DROP TABLE IF EXISTS my_table;

CREATE TABLE my_table
(id SERIAL PRIMARY KEY
,user_id INT NOT NULL
,subscribed_to VARCHAR(12) NOT NULL
,UNIQUE KEY(user_id,subscribed_to)
);

INSERT INTO my_table VALUES
(1,1,'English'),
(2,2,'English'),
(3,1,'Hindi'),
(4,3,'English'),
(5,3,'Hindi'),
(6,4,'English');

SELECT DISTINCT x.* 
  FROM my_table x 
  LEFT 
  JOIN my_table y 
    ON y.user_id = x.user_id 
   AND y.subscribed_to <> x.subscribed_to 
 WHERE x.subscribed_to = 'English' 
   AND y.id IS NULL;

+----+---------+---------------+
| id | user_id | subscribed_to |
+----+---------+---------------+
|  2 |       2 | English       |
|  6 |       4 | English       |
+----+---------+---------------+

Answer 2

假设subscribedto列中没有空值，则可以使用NOT EXISTS：

select t.*
from tablename t
where not exists (
  select 1 from tablename
  where userid = t.userid and subscribedto <> 'English'
)

请参见demo。
结果：

| id  | userid | subscribedto |
| --- | ------ | ------------ |
| 2   | 2      | English      |
| 5   | 4      | English      |

您还可以在条件子句中添加条件subscribedto = 'English'：

select t.*
from tablename t
where subscribedto = 'English' and not exists (
  select 1 from tablename
  where userid = t.userid and subscribedto <> 'English'
)

结果是相同的，但是此版本的效率可能更高。

Answer 3

分析功能将在单表扫描中执行此操作，无需昂贵的连接。它将在Hive中运行：

SELECT id, userid, subscribedto
FROM
(
SELECT id, userid, subscribedto,
       max(case when subscribedto != 'English' then true else false end) over(partition by userid ) subscribed_not_english
  FROM my_table s
)s 
WHERE NOT subscribed_not_english

如何从订阅列表中获得仅订阅英语频道的用户

3 个答案: