Question

I’ve worked with mysql for some time but have never had to do anything anywhere near complicated. I’m working on a new project and need a little push in the right direction. Lets say I want to have a table that stores cats. Lets say I want to store the cats name, and food that the cat likes. Then lets say I want to query all cats that like fish, milk, and mice. I don’t want to have the ‘cats’ table have rows for ‘fish’ ‘milk’ ‘mice’ that can be yes or no or 1 or 0. I think want I’m looking for is having a separate ‘foods’ table and then use the join statement. But I have no clue where to start looking or what to search for. Another way to think about this would be if you had a blog, and wanted posts to have categories. How would I store which posts are in which categories, and how would I query for posts by category?

I realize this is probably a very basic question, and would be happy with even a link to a tutorial explaining the structure / commands needed to pull this off! Thanks a lot!

Answer 1

Table: cats
id
name

Table: foods
id
name

Table: cat_food
cid  (Cat ID)
fid  (Food ID)

Query: Select cats that like fish
SELECT
name
FROM
cats c
INNER JOIN
cat_food cf
ON
cf.fid = 2 -- Assuming food ID#2 is 'fish'
Query: Select cats that like fish, milk, or mice
SELECT
c.name
FROM
cats c
INNER JOIN
cat_food cf
ON
cf.fid IN (1, 2, 3)
GROUP BY
c.name

SCHEMA
CREATE TABLE IF NOT EXISTS cats (
id int(10) unsigned NOT NULL AUTO_INCREMENT,
name varchar(255) COLLATE utf8_unicode_ci NOT NULL,
PRIMARY KEY (id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci AUTO_INCREMENT=3 ;

INSERT INTO cats (id, name) VALUES
(1, 'Sassy'),
(2, 'Tiger');

CREATE TABLE IF NOT EXISTS cat_food (
cid int(10) unsigned NOT NULL COMMENT 'Cat ID',
fid int(10) unsigned NOT NULL COMMENT 'Food ID',
UNIQUE KEY cid_fid (cid,fid),
KEY fid (fid)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

INSERT INTO cat_food (cid, fid) VALUES
(1, 1),
(2, 2),
(1, 3);

CREATE TABLE IF NOT EXISTS foods (
id int(10) unsigned NOT NULL AUTO_INCREMENT,
name varchar(255) COLLATE utf8_unicode_ci NOT NULL,
PRIMARY KEY (id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci AUTO_INCREMENT=4 ;

INSERT INTO foods (id, name) VALUES
(1, 'Fish'),
(2, 'Milk'),
(3, 'Mice');

ALTER TABLE cat_food
ADD CONSTRAINT cat_food_ibfk_2 FOREIGN KEY (fid) REFERENCES foods (id) ON DELETE CASCADE,
ADD CONSTRAINT cat_food_ibfk_1 FOREIGN KEY (cid) REFERENCES cats (id) ON DELETE CASCADE;