Sum hours value, count and display based on hours using SQL - mysql

Question

I have 2 tables which are Teacher and Activities.
CREATE TABLE teacher (
TeacherId INT, BranchId VARCHAR(5));
INSERT INTO teacher VALUES
("1121","A"),
("1132","A"),
("1141","A"),
("2120","B"),
("2122","B");
CREATE TABLE activities (
ID INT, TeacherID INT, Hours INT);
INSERT INTO activities VALUES
(1,1121,2),
(2,1121,1),
(3,1132,1),
(4,1141,NULL),
(5,2120,NULL),
(6,2122,NULL);
NULL indicates no activities and will be convert to 0 on output table. I want to produce a query to count total of hours and count how many activities base on teacher hours such as the following table:
+-----------+------------+------------+
| Hours | A | B |
+-----------+------------+------------+
| 0 | 1 | 2 |
| 1 | 1 | 0 |
| 2 | 0 | 0 |
| 3 | 1 | 0 |
+-----------+------------+------------+
Edited: Sorry I don't know how to elaborate accurately, but here is the fiddle i received from other member https://www.db-fiddle.com/f/mmtuZquKyUqdhPvTFN9qaF/1
Edit: Last, modification need, to sum the hours and count the hours base on branch id and teacher id as the output.
Expected output here (red text): https://drive.google.com/file/d/1wyZ_aX5hz_7I1Ncf5sXLpstYk6FT8PMg/view?usp=sharing

Answer 1

We can handle this via the use of a calendar table of hours joined to an aggregation subquery:
SELECT
t1.Hours,
SUM(CASE WHEN t2.BranchId = 'A' THEN t2.cnt ELSE 0 END) AS A,
SUM(CASE WHEN t2.BranchId = 'B' THEN t2.cnt ELSE 0 END) AS B
FROM (SELECT 0 AS Hours UNION ALL SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3) t1
LEFT JOIN
(
SELECT t.BranchId, COALESCE(a.Hours, 0) AS Hours, COUNT(*) AS cnt
FROM Teacher t
LEFT JOIN Activities a ON a.TeacherId = t.TeacherId
GROUP BY t.BranchId, COALESCE(a.Hours, 0)
) t2
ON t1.Hours = t2.Hours
GROUP BY
t1.Hours
ORDER BY
t1.Hours
Demo

Answer 2

This is basically a JOIN and aggregation . . . but you need to start with all the hours you want:
SELECT h.Hours,
COALESCE(SUM(t.BranchId = 'A'), 0) AS A,
COALESCE(SUM(t.BranchId = 'B'), 0) AS B
FROM (SELECT 0 AS Hours UNION ALL SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3
) h LEFT JOIN
activities a
ON h.hours = COALESCE(a.hours, 0) LEFT JOIN
teacher t
ON t.TeacherId = a.TeacherId
GROUP BY h.Hours
ORDER BY h.Hours;
Here is a db<>fiddle.