How to join multiple count(*) queries? - mysql

Question

Is there a way to join multiple count(*) queries to a single one? I know that i don't need to prepare the last two queries two times but that's not what the question is about. I have no idea how to do this in a correct way.
$sth = $this->_db->prepare('SELECT count(*) FROM posts WHERE topicid = ?');
$sth->execute( array( $this->_id ) );
$numPosts = $sth->fetch(\PDO::FETCH_ASSOC);
$sth = $this->_db->prepare('SELECT count(*) FROM topic_activity WHERE topicid = ?');
$sth->execute( array( $this->_id ) );
$numViews = $sth->fetch(\PDO::FETCH_ASSOC);
$sth = $this->_db->prepare('SELECT count(*) FROM topic_activity WHERE topicid = ? AND likes = 1');
$sth->execute( array( $this->_id ) );
$numLikes = $sth->fetch(\PDO::FETCH_ASSOC);
$sth = $this->_db->prepare('SELECT count(*) FROM topic_activity WHERE topicid = ? AND likes = -1');
$sth->execute( array( $this->_id ) );
$numDislikes = $sth->fetch(\PDO::FETCH_ASSOC);

Answer 1

SELECT count_post, topic_activity
FROM(
SELECT count(*) AS count_post, NULL AS topic_activity FROM posts WHERE topicid = ?
UNION
SELECT NULL AS count_post, count(*) AS topic_activity FROM topic_activity WHERE topicic =?
) T
GROUP by count_post, topic_activity
let me know if it works

Answer 2

Although the answer of user3106988 'works', I would have gone for a more 'vertical' approach (and also store the id in a variable so you only need to pass it once, could be this doesn't work in your environment though, so could be you'll need to replace the #id's with 4 question marks, but hopefully not) :
SET #id = ?;
SELECT info = 'Posts', count(*) FROM posts WHERE topicid = #id
UNION ALL
SELECT info = 'Views', count(*) FROM topic_activity WHERE topicid = #id
UNION ALL
SELECT info = 'Likes', count(*) FROM topic_activity WHERE topicid = #id AND likes = 1
UNION ALL
SELECT info = 'DisLikes', count(*) FROM topic_activity WHERE topicid = #id AND likes = -1
But that returns 4 records where you'll then need to extract the data out by means of the info field. So, building on the idea of user3106988's answer to return just 1 record with all the info in different fields you could actually go for the likes & disklikes in as little effort as below. Should be quite fast.
SET #id = ?;
SELECT SUM(count_posts) as count_posts,
SUM(views) as views,
SUM(likes) as likes,
SUM(dislikes) as dislikes
FROM(
SELECT COUNT(*) as count_posts,
0 as views,
0 as likes,
0 as dislikes
FROM posts
WHERE topicid = #id
UNION ALL
SELECT 0 as count_posts,
COUNT(*) as views,
SUM(Case WHEN likes = 1 THEN 1 ELSE 0 END) as likes,
SUM(Case WHEN likes = -1 THEN 1 ELSE 0 END) as dislikes
FROM topic_activity
WHERE topicid = #id
) T;