What is wrong with my JOINs? - mysql

Question

So I am incredibly new to MySQL and I am struggling every step of the way to learning it. I am supposed to create a small database that can be about anything for my class assignment. I have the tables created but I am trying to figure out how to correctly JOIN them together.
Here is the code I currently have for it:
CREATE TABLE `monster` (
`monster_id` INTEGER NULL AUTO_INCREMENT DEFAULT NULL,
`name` VARCHAR(30) NULL DEFAULT NULL,
`hitpoints` TINYINT NULL DEFAULT NULL,
`armorclass` TINYINT NULL DEFAULT NULL,
PRIMARY KEY (`monster_id`)
);
CREATE TABLE `monster_size` (
`monster_id` INTEGER NULL AUTO_INCREMENT DEFAULT NULL,
`size_id` INTEGER NULL DEFAULT NULL,
PRIMARY KEY (`monster_id`)
);
CREATE TABLE `size` (
`size_id` INTEGER NULL AUTO_INCREMENT DEFAULT NULL,
`size` VARCHAR(15) NULL DEFAULT NULL,
PRIMARY KEY (`size_id`)
);
ALTER TABLE `monster_size` ADD FOREIGN KEY (monster_id) REFERENCES `monster` (`monster_id`);
ALTER TABLE `monster_size` ADD FOREIGN KEY (size_id) REFERENCES `size` (`size_id`);
/* Here is where I am trying to get the joining to happen */
SELECT name,hitpoints,size
FROM monster m
JOIN size s ON (m.monster_id=s.size_id)
JOIN monster_size ms ON (s.size_id=ms.size_id);

Answer 1

You're not joining the tables in the correct way. You'll want to join the monster table with the monster_size junction table, and then the size table with the monster_size table, like this:
SELECT name,hitpoints,size
FROM monster m
JOIN monster_size ms ON m.monster_id = ms.monster_id
JOIN size s ON ms.size_id = s.size_id
In your current query you're trying to match the id from the size table with the id from the monster table, but those two tables doesn't have any direct relation, so you need to use the monster_size table to connect them.

Answer 2

You are using a linking table monster_size. This means that you are creating an m to n relation. Can a monster really have several sizes at the same time? Because now, a size can be linked to several monsters, but a monster can also be linked to several sizes through this linking table.
Also this linking table should not have an auto increment column.
But probably you just want to link one size to a monster at a time. Forget the monster_size table. Instead add a size_id column to the monster table (no auto increment). Probably the set of possible sizes will not change anyway, so you could drop the auto increment in the size table as well and add well defines IDs "manually".
New table structure
DROP TABLE IF EXISTS `monster`;
CREATE TABLE `monster` (
`monster_id` INTEGER NULL AUTO_INCREMENT DEFAULT NULL,
`name` VARCHAR(30) NULL DEFAULT NULL,
`hitpoints` TINYINT NULL DEFAULT NULL,
`armorclass` TINYINT NULL DEFAULT NULL,
`size_id` INTEGER NULL DEFAULT NULL,
PRIMARY KEY (`monster_id`)
);
DROP TABLE IF EXISTS `size`;
CREATE TABLE `size` (
`size_id` INTEGER NULL DEFAULT NULL,
`size` VARCHAR(15) NULL DEFAULT NULL,
PRIMARY KEY (`size_id`)
);
ALTER TABLE `monster` ADD FOREIGN KEY (size_id) REFERENCES `size` (`size_id`);
INSERT INTO size (size_id, size) VALUES (1, 'small');
INSERT INTO size (size_id, size) VALUES (2, 'big');
INSERT INTO size (size_id, size) VALUES (3, 'huge');
INSERT INTO monster(name, hitpoints, size_id) VALUES ('Tester', 10, 1);
Now, your query goes like this
SELECT m.name, m.hitpoints, s.size
FROM
monster m
LEFT JOIN size s
ON (m.size_id = s.size_id);
Also use a left outer join, in case a size_id is null in the monster table.