How to count correctly 2 inner levels count? - mysql

Question

I have a structure for a university DB wherein I have three tables: room, students, possessions
CREATE TABLE `rooms` (
`id` INT(10) NOT NULL AUTO_INCREMENT,
`name` VARCHAR(50) NOT NULL COLLATE 'utf8_general_ci',
PRIMARY KEY (`id`) USING BTREE,
UNIQUE INDEX `name` (`name`) USING BTREE
)
COLLATE='utf8_general_ci'
ENGINE=InnoDB
AUTO_INCREMENT=3
;
CREATE TABLE `students` (
`id` INT(10) NOT NULL AUTO_INCREMENT,
`ref_room_id` INT(10) NULL DEFAULT NULL,
`student_name` VARCHAR(50) NOT NULL COLLATE 'utf8_general_ci',
PRIMARY KEY (`id`) USING BTREE,
INDEX `FK_students_room` (`ref_room_id`) USING BTREE,
CONSTRAINT `FK_students_room` FOREIGN KEY (`ref_room_id`) REFERENCES `university`.`rooms` (`id`) ON UPDATE CASCADE ON DELETE SET NULL
)
COLLATE='utf8_general_ci'
ENGINE=InnoDB
AUTO_INCREMENT=6
;
CREATE TABLE `possessions` (
`id` INT(10) NOT NULL AUTO_INCREMENT,
`ref_student_id` INT(10) NOT NULL,
`name` VARCHAR(50) NOT NULL DEFAULT '' COLLATE 'utf8_general_ci',
PRIMARY KEY (`id`) USING BTREE,
INDEX `FK__students` (`ref_student_id`) USING BTREE,
CONSTRAINT `FK__students` FOREIGN KEY (`ref_student_id`) REFERENCES `university`.`students` (`id`) ON UPDATE NO ACTION ON DELETE NO ACTION
)
COLLATE='utf8_general_ci'
ENGINE=InnoDB
AUTO_INCREMENT=4
;
INSERT INTO rooms (name) VALUES ('a'),('b');
INSERT INTO students (`ref_room_id`, `student_name`) VALUES
(3,1),
(3,2),
(3,3),
(3,4);
INSERT INTO possessions (ref_student_id, name) VALUES
(7,'a')
(7,'aa'),
(7,'aaa'),
(8,'aaaa'),
(9,'aaaaa'),
(6,'aaaaaa'),
(7,'aaaaaaa');
So, in order to present such a table in MySQL, I created the procedure
CREATE DEFINER=`root`#`localhost` PROCEDURE `get_data`()
LANGUAGE SQL
NOT DETERMINISTIC
CONTAINS SQL
SQL SECURITY DEFINER
COMMENT ''
BEGIN
SELECT r.id AS room_id, r.name, COUNT(s.id) AS student_num, COUNT(p.id) AS possessions_num
FROM rooms r
INNER JOIN students s ON s.ref_room_id = r.id
INNER JOIN possessions p ON p.ref_student_id = s.id
GROUP BY r.name;
END
but what I get is
First of all, it misses the room and secondly instead of 4 students it shows 7...
What am I doing wrong?

Answer 1

Consider the following (and note that my data set may vary slightly from yours...)
DROP TABLE IF EXISTS rooms;
CREATE TABLE `rooms` (
`id` INT(10) NOT NULL AUTO_INCREMENT,
`name` VARCHAR(50) NOT NULL ,
PRIMARY KEY (`id`)
);
DROP TABLE IF EXISTS students;
CREATE TABLE `students` (
`id` INT(10) NOT NULL AUTO_INCREMENT,
`ref_room_id` INT(10) NULL DEFAULT NULL,
`student_name` VARCHAR(50) NOT NULL,
PRIMARY KEY (`id`)
);
DROP TABLE IF EXISTS possessions;
CREATE TABLE `possessions` (
`id` INT NOT NULL AUTO_INCREMENT,
`ref_student_id` INT NOT NULL,
`name` VARCHAR(50) NOT NULL,
PRIMARY KEY (`id`)
);
INSERT INTO rooms VALUES (1,'a'),(2,'b');
INSERT INTO students VALUES
(11,1,1),
(12,1,2),
(13,1,3),
(14,1,4);
INSERT INTO possessions VALUES
(101,11,'apple'),
(102,11,'banana'),
(103,11,'cherry'),
(104,12,'date'),
(105,15,'elderberry'),
(106,13,'fig'),
(107,14,'huckleberry');
SELECT r.*
, COUNT(DISTINCT s.id) total_students
, COUNT(DISTINCT p.id) total_possessions
FROM rooms r
JOIN students s
ON s.ref_room_id = r.id
JOIN possessions p
ON p.ref_student_id = s.id
GROUP
BY r.id;
+----+------+----------------+-------------------+
| id | name | total_students | total_possessions |
+----+------+----------------+-------------------+
| 1 | a | 4 | 6 |
+----+------+----------------+-------------------+
Or...
SELECT r.*
, COUNT(DISTINCT s.id) total_students
, COUNT(DISTINCT p.id) total_possessions
FROM rooms r
LEFT
JOIN students s
ON s.ref_room_id = r.id
LEFT
JOIN possessions p
ON p.ref_student_id = s.id
GROUP
BY r.id;
+----+------+----------------+-------------------+
| id | name | total_students | total_possessions |
+----+------+----------------+-------------------+
| 1 | a | 4 | 6 |
| 2 | b | 0 | 0 |
+----+------+----------------+-------------------+