如何从订阅列表中获取仅订阅英语频道的用户

Posted 2023-03-23

【中文标题】如何从订阅列表中获取仅订阅英语频道的用户

【英文标题】：From a subscribed list how get users who only subscribed to English channels

【发布时间】：2019-07-26 16:46:09

【问题描述】：

我的表格有用户订阅英语和印地语频道的记录，我只想要订阅英语的用户

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

所以结果会是

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

【问题讨论】：

为什么结果表中没有id 1和4？他们还订阅了英语

你可以做select * from YOUR_TABLE_NAME where Subscribedto='English';

我想你的意思是“只订阅英语”

【参考方案1】：

假设subscribedto 列中没有空值，您可以使用 NOT EXISTS：

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

请参阅demo。 结果：

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

您还可以在 where 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'
)

结果是一样的，但这个版本可能会更高效。

【参考方案2】：

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       |
+----+---------+---------------+

【参考方案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 中工作：

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