MySQL COUNT and GROUP BY subquery - mysql

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

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 2

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 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