Three table join with matched and unmatched records

Question

I have three tables:

• store
• article_to_store
• article table.

    CREATE TABLE store (
    `id` int(10) unsigned NOT NULL,
    `name` varchar(20) NOT NULL,
    PRIMARY KEY (`id`)
    ) ENGINE=InnoDB DEFAULT CHARSET=latin1;

    CREATE TABLE article_to store (
    `id` int(10) unsigned NOT NULL,
    `article_id` int(10) unsigned NOT NULL,
    `store_id` int(10) unsigned NOT NULL,
    `Price`....,
    PRIMARY KEY (`id`)
    ) ENGINE=InnoDB DEFAULT CHARSET=latin1;
    
    CREATE TABLE article (
        `id` int(10) unsigned NOT NULL,
        `name` varchar(20) NOT NULL,
        PRIMARY KEY (`id`)
        ) ENGINE=InnoDB DEFAULT CHARSET=latin1;

I need all articles with the store name. For all articles, I want to list all stores

store table contains 3 stores. article table contains 2 articles.

article_to_store relation table maybe has 2 records. One article in 2 stores.

I need a mysql query with this result:

store.name	article_to_store.id	article.name
Store 1	1	Pen
Store 2	2	Pen
Store 3	null	Pen
Store 1	null	Erase
Store 2	null	Erase
Store 3	null	Erase

SELECT store.name, article.name, article_to_store.id 

FROM store, article_to_store, article 

Group by store.id, article.id
ORDER BY article.id, store.id

If I use simple join the article_to_store table always id = 1

If I use LEFT JOIN, don't get the unmatched records.

When I remove the group by then I got 12 records.

3 stores * 2 article_to_store * 2 article = 12 record. Thats not good for me.

I would like to 3 stores * 2 article = 6 record with article_to store data if exist.

I tried many join types, but it doesn't work. Thanks for any ideas.

Answer 1

You can use a cross join to achieve this.

SELECT s.name, ats.id, a.name
FROM article a
CROSS JOIN store s
LEFT JOIN article_to_store ats ON a.id = ats.article_id AND s.id = ats.store_id
ORDER BY a.id, s.id

A bit of an unusual scenario, but here, you're essentially asking "For every article, I want to list all stores - that's a cross join ". Then to get the middle column, you're only sometimes expecting a row back, which is a left join .