MySQL COUNT and GROUP BY subquery

Question

I'm trying to find the count of posts grouped by branch and category. I'm not getting the categories with count 0.

CREATE TABLE branches
    (`id` serial primary key, `name` varchar(7) unique)
;

INSERT INTO branches
    (`id`, `name`)
VALUES
    (1, 'branch1'),
    (2, 'branch2'),
    (3, 'branch3')
;


CREATE TABLE categories
    (`id` serial primary key, `category` varchar(4) unique)
;

INSERT INTO categories
    (`id`, `category`)
VALUES
    (1, 'cat1'),
    (2, 'cat2')
;


CREATE TABLE posts
    (`id` serial primary key, `branch_id` int, `category_id` int, `title` varchar(6), `created_at` varchar(10))
;

INSERT INTO posts
    (`id`, `branch_id`, `category_id`, `title`, `created_at`)
VALUES
    (1, 1, 1, 'Title1', '2017-12-14'),
    (2, 1, 2, 'Title2', '2018-01-05'),
    (3, 2, 1, 'Title3', '2018-01-10')
;

Expected Output:

+---------+----------+----+----+
| branch  | category | c1 | c2 |
+---------+----------+----+----+
| branch1 | cat1     |  1 |  0 |
| branch1 | cat2     |  0 |  1 |
| branch2 | cat1     |  0 |  1 |
| branch2 | cat2     |  0 |  0 |
+---------+----------+----+----+

Query tried:

SELECT b.name, x.c1, y.c2 FROM branches b
LEFT JOIN (
     SELECT COUNT(id) c1 FROM posts WHERE created_at < '2018-01-01'
     GROUP BY posts.branch_id, posts.category_id
) x x.branch_id = b.id
LEFT JOIN (
     SELECT COUNT(id) c2 FROM posts WHERE created_at BETWEEN '2018-01-01' AND '2018-01-31'
     GROUP BY posts.branch_id, posts.category_id
) y y.branch_id = b.id
GROUP BY b.id

Answer 1

It looks like this might do what you want.

Explanation: Get each possible combination of branch/category for branches which exists in posts. Do a conditional sum to get the counts by date range and branch/category. Then join back to branch.

SELECT b.b_id branch,
       b.category,
       COALESCE(Range_Sum.C1,0) C1,
       COALESCE(Range_Sum.C2,0) C2
  FROM ( SELECT b.id b_id,
                c.id c_id,
                c.category
           FROM branches b,
                categories c
          WHERE EXISTS
                  ( SELECT 1
                      FROM posts
                     WHERE b.id = posts.branch_id
                  )
       ) b
  LEFT  
  JOIN (SELECT p.branch_id,
               c.id c_id,
               c.category,
               SUM
                 ( CASE WHEN p.created_at < '2018-01-01' THEN 1 
                        ELSE 0
                    END
                 ) C1,
               SUM
                 ( CASE WHEN p.created_at BETWEEN '2018-01-01' AND '2018-01-31' THEN 1
                        ELSE 0
                    END
                 ) C2
          FROM posts p
         INNER
          JOIN categories c
            ON p.category_id = c.id
         GROUP
            BY p.branch_id,
               c.category,
               c.id
       ) Range_Sum
    ON b.b_id = Range_Sum.branch_id
   AND b.c_id = Range_Sum.c_id;

Also, just a thing for writing easily readable queries - NEVER use x and y as aliases. Choose anything else that could possibly be more informative.

Answer 2

You need to CROSS JOIN branches and categories first; then LEFT JOIN to posts and do conditional counts based on your WHERE criteria.

Generic format:

SELECT x.data, y.data
, COUNT(CASE WHEN conditionN THEN 1 ELSE NULL END) AS cN
FROM x CROSS JOIN y
LEFT JOIN z ON x.id = z.x_id AND y.id = z.y_id
GROUP BY x.data, y.data
;

Note: COUNT (and pretty much all aggregate functions) ignore NULL values.

Answer 3

Maybe a little contrived...

SELECT DISTINCT x.branch_id
              , y.category_id
              , COALESCE(z.created_at < '2018-01-01',0) c1
              , COALESCE(z.created_at BETWEEN '2018-01-01' AND '2018-01-31',0) c2
           FROM posts x
           JOIN posts y
           LEFT
           JOIN posts z
             ON z.branch_id = x.branch_id
            AND z.category_id  = y.category_id;

http://sqlfiddle.com/#!9/8aabf2/31