SQL split column by comma in where clause - mysql

Question

I am trying to display where a record has multiple categories though my query only appears to be showing the first instance. I need for the query to be displaying the domain multiple times for each category it appears in
The SQL statement I have is
SELECT domains.*, category.*
FROM domains,category
WHERE category.id IN (domains.category_id)
Which gives me the below results

Answer 1

You should not store numeric values in a string. Bad, bad idea. You should use a proper junction table and the right SQL constructs.
Sometimes, we are stuck with other people's bad design decisions. MySQL offers find_in_set() to help in this situation:
where find_in_set(category.id, domains.category_id) > 0

Answer 2

Use find_in_set().
SELECT domains.*, category.*
FROM domains,category
WHERE find_in_set (category.id ,domains.category_id)
But it is very bad db design to store fk as a csv.

Answer 3

As others have pointed out, you can use FIND_NI_SET() as a workaround to solve your problem.
My suggestion is that you refactor your code and database a bit. Storing values in CSV format stored in a single column is almost always a bad idea.
As Gordon Linoff correctly points out you'd be better off if you'd create an additional table to store the category_id values:
CREATE TABLE domain_categories (domain_id INT, category_id INT, PRIMARY KEY (domain_id, category_id));
That's assuming you want to enfore that each category is only stored once against each domain. If you don't, just drop the primary key.
You'd then insert your IDs into this new table:
INSERT INTO domain_categories (domain_id, category_id) VALUES (2,6),(2,8);
or
INSERT INTO domain_categories (domain_id, category_id) VALUES (4,3);
INSERT INTO domain_categories (domain_id, category_id) VALUES (4,11);
INSERT INTO domain_categories (domain_id, category_id) VALUES (20,3);
Now that you have properly stored the data you can easily query as needed:
SELECT domains.*,category,*
FROM domains
JOIN domain_category ON (domain_category.domain_id=domains.id)
JOIN category ON (category.id=domain_category.category_id);
Using MySQL quirks you can even show the category_id column in CSV format.
SELECT domains.*, GROUP_CONCAT(DISTINCT domain_category.category_id)
FROM domains
JOIN domain_category ON (domain_category.domain_id=domains.id)
GROUP BY domains.id;