Roys-MacBook-Pro:likedemo roy$ cat likes.csv 
Id,Postid,Userid
1,1,2
1,1,1
1,1,3
1,2,2
Roys-MacBook-Pro:likedemo roy$ cat posts.csv 
Id,Name,Content,Author
1,Hello world,Is my friend,Matti
2,Hello earth,Is my friend,Matti
3,Hello venus,Is not my friend,Jussi
Roys-MacBook-Pro:likedemo roy$ cat users.csv 
Id,Name,Password
1,tester1,hunter2
2,tester2,hunter2
3,tester3,hunter2
4,tester4,hunter2
5,tester5,hunter2
Roys-MacBook-Pro:likedemo roy$ cat get_posts_w_likes.sql 
SELECT
    POSTS.Name,
    (SELECT COUNT(Id) FROM `likes.csv` AS LIKES WHERE LIKES.Postid = POSTS.id)
FROM
    `posts.csv` AS POSTS
Roys-MacBook-Pro:likedemo roy$ csvq -s get_posts_w_likes.sql 
+--------------+----------------------------------------------------------------------------+
|     Name     | (SELECT COUNT(Id) FROM `likes.csv` AS LIKES WHERE LIKES.Postid = POSTS.id) |
+--------------+----------------------------------------------------------------------------+
| Hello world  |                                                                          3 |
| Hello earth  |                                                                          1 |
| Hello venus  |                                                                          0 |
+--------------+----------------------------------------------------------------------------+
Roys-MacBook-Pro:likedemo roy$