来自带有条件的许多INNER JOIN表的多个计数

Question

I'm having a lot of trouble figuring out how to write this query. Here's an exmaple of the data set and what I need to query:

**System Table**    

SystemID   Active
1          T
2          T
3          T
4          F
5          F
6          F

**BlogPost Table**      

BlogPostID  SystemID    Create_Month
100         2           Jan
101         2           Jan
102         2           Feb
103         3           Feb
104         3           Mar
105         6           Mar
106         6           Mar

**Comment Table**       

Comment ID  BlogPostID  Liked
201         100         T
202         100         T
203         100         T
204         102         T
205         102         T
206         102         T
207         103         F

So, In words, I'm trying to get: By month, show me all the active systems who created a post during that month, the number of posts they made in aggregate, and the count of the subset of those posts who had a comment that was like.

The end result would be like:

Column 1 - Month

Column 2 - Count of Active Systems where a Post Created in Month

Column 3 - Count of Posts Applicable to those systems

Column 4 - Count of Applicable Posts that had comments that were liked  

I don't even know where to start really. My terrible "this is obviously wrong" attempt is below. Any help is much appreciated, thanks!

SELECT
  Month, 
  COUNT(DISTINCT system.systemid), 
  COUNT(blogpost.BlogPostID)
  COUNT(comments.commentiD)
FROM
  system INNER JOIN
  blogpost ON system.systemid = blogpost.systemid INNER JOIN
  comments ON blogpost.BlogPostID = comment.BlogPostID
WHERE
  system.active = T 
  AND comments.like = T
GROUP BY month

Answer 1

A complicated one !

SELECT 
b.Create_Month, 
COUNT(DISTINCT s.SystemID) as SystemCount, 
COUNT(DISTINCT b.BlogPostID) as PostsCount,
COUNT(DISTINCT t.BlogPostID) as PostsWithLike

FROM System s 

JOIN BlogPost b 
ON s.systemID = b.systemID 
AND s.Active = 'T'  

LEFT JOIN Comment c 
ON b.BlogPostID = c.BlogPostID

LEFT JOIN
(
SELECT DISTINCT c.BlogPostID as BlogPostID
FROM 
Comment c 
GROUP BY c.BlogPostID 
HAVING SUM(if(c.Liked='T',1,0))>0
) as t
ON b.BlogPostID = t.BlogPostID

GROUP BY b.Create_Month 

Answer 2

This is probably what you want :

SELECT s.systemid, active, bp.create_month, bp.systemid, COUNT(bp.blogpostid), COUNT(c.liked)
FROM system AS s 
     LEFT OUTER JOIN Blogpost AS bp ON s.systemid = bp.systemid 
     LEFT OUTER JOIN Comment AS c ON bp.blogpostid = c.blogpostid 
WHERE active = 'T' AND c.Liked = 'T' GROUP BY s.systemid,bp.create_month