如何通过组与SQL子查询

Question

我无法想清楚的那一刻，我想station_id返回计数，输出的一个例子是：

站1有3个FB后，6个LinkedIn帖子，5个电子邮件帖子 站2有3个FB后，6级LinkedIn的职位，5个电子邮件帖子

所以我需要组由台ID，我的表结构

CREATE TABLE IF NOT EXISTS `posts` (
    `post_id` bigint(11) NOT NULL auto_increment, 
    `station_id` varchar(25) NOT NULL, 
    `user_id` varchar(25) NOT NULL, 
    `dated` datetime NOT NULL, 
    `type` enum('fb','linkedin','email') NOT NULL, 
    PRIMARY KEY (`post_id`) 
) ENGINE=MyISAM DEFAULT CHARSET=latin1 AUTO_INCREMENT=x ; 

查询我至今当它有一个（2分贝t浩）

SELECT Station_id, (select count(*) FROM posts WHERE type = 'linkedin') AS linkedin_count, (select count(*) FROM posts WHERE type = 'fb') AS fb_count, (select count(*) FROM posts WHERE type = 'email') AS email_count FROM `posts` GROUP BY station_id; 

Answer 1

或者，以最快的方式，避免了连接和子查询得到它的确切格式你想：

SELECT 
    station_id, 
    SUM(CASE WHEN type = 'linkedin' THEN 1 ELSE 0 END) AS 'linkedin', 
    SUM(CASE WHEN type = 'fb'  THEN 1 ELSE 0 END) AS 'fb', 
    SUM(CASE WHEN type = 'email' THEN 1 ELSE 0 END) AS 'email' 
FROM posts 
GROUP BY station_id; 

输出：

+------------+----------+------+-------+ 
| station_id | linkedin | fb | email | 
+------------+----------+------+-------+ 
| 1   |  3 | 2 |  5 | 
| 2   |  2 | 0 |  1 | 
+------------+----------+------+-------+ 

你可能还想在那里放一个索引来加速它

ALTER TABLE posts ADD INDEX (station_id, type); 

Explain输出：

+----+-------------+-------+-------+---------------+------------+---------+------+------+-------------+ 
| id | select_type | table | type | possible_keys | key  | key_len | ref | rows | Extra  | 
+----+-------------+-------+-------+---------------+------------+---------+------+------+-------------+ 
| 1 | SIMPLE  | posts | index | NULL   | station_id | 28  | NULL | 13 | Using index | 
+----+-------------+-------+-------+---------------+------------+---------+------+------+-------------+ 

Answer 2

试试这个：

SELECT p.Station_id, 
(select count(*) FROM posts WHERE type = 'linkedin' and station_id=p.station_id) AS linkedin_count, 
(select count(*) FROM posts WHERE type = 'fb' and station_id=p.station_id) AS fb_count, 
(select count(*) FROM posts WHERE type = 'email' and station_id=p.station_id) AS email_count 
FROM `posts` p GROUP BY station_id 

Answer 3

给这个一展身手：

SELECT station_id, type, count(*) FROM posts GROUP BY station_id, type 

输出格式将是你的企图变得有点不同，但它应该提供您尝试检索的统计信息。另外，因为它的单个查询速度更快。

- 编辑，添加例如结果集

+------------+----------+----------+ 
| station_id | type  | count(*) | 
+------------+----------+----------+ 
| 1   | fb  |  2 | 
| 1   | linkedin |  3 | 
| 1   | email |  5 | 
| 2   | linkedin |  2 | 
| 2   | email |  1 | 
+------------+----------+----------+ 

Answer 4

正如所暗示的gnif的回答，有三个相关sub_queries有过头的性能。根据您使用的DBMS，它可以执行类似于自我连接三次的操作。

gnif的方法确保该表只解析一次，而无需连接的需要，相关sub_queries等

的立即下侧gnif的回答很明显的是，你永远不得到记录0的。如果没有fb类型，你只是不会得到一个记录。如果这不是问题，我会回答他的答案。如果它是一个问题，但是，这里是类似的方法一个版本gnif，但你的输出格式...

SELECT 
    station_id, 
    SUM(CASE WHEN type = 'linkedin' THEN 1 ELSE 0 END) AS linkedin_count, 
    SUM(CASE WHEN type = 'fb'  THEN 1 ELSE 0 END) AS fb_count, 
    SUM(CASE WHEN type = 'email' THEN 1 ELSE 0 END) AS email_count 
FROM 
    posts 
GROUP BY 
    station_id