how to group by with a sql subqueries - mysql

Question

I can't think clearly at the moment, I want to return counts by station_id, an example of output would be:
station 1 has 3 fb post, 6 linkedin posts, 5 email posts
station 2 has 3 fb post, 6 linkedin posts, 5 email posts
So I need to group by the station id, my table structure is
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 ;
The query I have so far is returning station 0 as having 2 linkedin posts when it has one (2 in the db tho)
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

Or, the fastest way, avoiding joins and subselects to get it in the exact format you want:
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;
Outputs:
+------------+----------+------+-------+
| station_id | linkedin | fb | email |
+------------+----------+------+-------+
| 1 | 3 | 2 | 5 |
| 2 | 2 | 0 | 1 |
+------------+----------+------+-------+
You may also want to put an index on there to speed it up
ALTER TABLE posts ADD INDEX (station_id, type);
Explain output:
+----+-------------+-------+-------+---------------+------------+---------+------+------+-------------+
| 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

As implied by gnif's answer, having three correlated sub_queries has a performance over-head. Depending on the DBMS you're using, it could perform similarly to having a self join three times.
gnif's methodology ensures that the table is only parsed once, without the need for joins, correlated sub_queries, etc.
The immediately obvious down-side of gnif's answer is that you don't ever get records for 0's. If there are no fb types, you just don't get a record. If that is not an issue, I'd go with his answer. If it is an issue, however, here is a version with similar methodology to gnif, but matching your output format...
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

Answer 3

Give this a go:
SELECT station_id, type, count(*) FROM posts GROUP BY station_id, type
The output format will be a little different to what your attempting to get, but it should provide the statistics your trying to retrieve. Also since its a single query it is much faster.
-- Edit, added example result set
+------------+----------+----------+
| station_id | type | count(*) |
+------------+----------+----------+
| 1 | fb | 2 |
| 1 | linkedin | 3 |
| 1 | email | 5 |
| 2 | linkedin | 2 |
| 2 | email | 1 |
+------------+----------+----------+

Answer 4

try this:
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