MySQL - using results of multiple subqueries with un-matching numbers of rows

Question

I don't know if this is possible but I'm trying to use the value from a MySQL SELECT query in multiple subqueries following the advice given at How to specify the parent query field from within a subquery in mySQL? . There are unequal numbers of results in each subquery, which I suspect is why I can't get this working.

My DB is more complex than on the above post. I'm dealing with 5 tables which are as follows:

1. substances - a list of chemical substances, all of which have a unique ID ( substances.id )
2. ecs - EC Number* values.
3. cas - CAS Number* values.
4. ecs_substances - Mapping substances.id to ecs.id . One substances.id may be mapped to ecs.id ecs.id
5. cas_substances - Mapping substances.id to cas.id . One substances.id may be mapped to cas.id cas.id

* These are terms used for classifying chemicals.

An example of how the data may look for 1 substance:

substances.id  | substances.name
-------------- | ----------------
1              | FooBar

An example of the assignment to multiple CAS numbers (the same principle applies for EC but am just using CAS for brevity):

cas_substances.id | substance_id
--------------------------------
997               | 1
----------------- | ------------
23423             | 1
--------------------------------

And then the CAS Numbers themselves:

cas.id | cas.value
------ | ---------
997    | ABC-123
------ | ---------
23423  | XYZ-876
------------------

The desired result is that I want to obtain a list of each substances.id, substances.name with a corresponding list of the CAS/EC Numbers, ideally formatted with a line break, eg

substances.id  | substances.name | cas_values         |
-------------- | --------------- | ------------------ |
1              | FooBar          | ABC-123 "\n" XYZ-876

I understand that the logic to be as follows:

1. SELECT * FROM substances
2. SELECT * FROM cas_substances WHERE cas_substances.substance_id = Result(s) from 1 ( substances.id )
3. SELECT cas.value FROM cas WHERE cas.cas.id = Result(s) from 2 ( cas_substances.cas_id )
4. Formatting to handle the fact there may (or may not) be multiple cas_substances rows - line break between records if there is.

I don't understand how to write the SQL that gets Results from 1/2 .

Is it even possible to do this in MySQL? The application I'm building is written in PHP but I figured using SQL only will be far quicker than doing multiple subqueries in PHP (although it is much easier to write in PHP).

Answer 1

You can join these three tables together, then use group by and group_concat to achieve that.

select
    `sub`.*,
    group_concat(cas.`value` separator ' "\n" ') as cas_values
from substances `sub`
left join cas_substances cs on `sub`.id = cs.substance_id
left join cas on cas.id = cs.cas_id
group by `sub`.id

A demo here.

Note: GROUP_CONCAT only works in mysql.

Answer 2

The accepted answer works but I have expanded this to include all 5 tables:

select
`sub`.*,
group_concat(cas.`value` separator ' "\n" ') as cas_values,
group_concat(ecs.`value` separator ' "\n" ') as ecs_values
from substances `sub`
left join cas_substances cs on `sub`.id = cs.substance_id
left join cas on cas.id = cs.cas_id
left join ecs_substances ec on `sub`.id = ec.substance_id
left join ecs on ecs.id = ec.ec_id
group by `sub`.id