Vertically concat MySQL tables, inserting NULLs - mysql

Question

I have a table mmm with main_ids, and 2 tables (ppp and ccc) with main_ids and sub-IDs, like this
Table Structure
Table `mmm` Table `ppp` Table `ccc`
+---------+ +---------+--------+ +---------+--------+
| main_id | | main_id | ppp_id | | main_id | ccc_id |
+---------+ +---------+--------+ +---------+--------+
| 1 | | 1 | 1 | | 3 | 1 |
| 2 | | 2 | 2 | | 4 | 2 |
| 3 | +---------+--------+ +---------+--------+
| 4 |
+---------+
The cccs and the ppps get their main_ids from the pool of main_ids in mmm.
Commands to construct
CREATE DATABASE test;
USE test;
CREATE TABLE mmm (main_id INT, PRIMARY KEY (main_id));
CREATE TABLE ppp (main_id INT, ppp_id INT, FOREIGN KEY (main_id) REFERENCES mmm (main_id));
CREATE TABLE ccc (main_id INT, ccc_id INT, FOREIGN KEY (main_id) REFERENCES mmm (main_id));
INSERT INTO mmm VALUES (1), (2), (3), (4);
INSERT INTO ppp VALUES (1, 1), (2, 2);
INSERT INTO ccc VALUES (3, 1), (4, 2);
I want to write a query that produces this result:
Desired Output
+---------+--------+--------+
| main_id | ppp_id | ccc_id |
+---------+--------+--------+
| 1 | 1 | NULL |
| 2 | 2 | NULL |
| 3 | NULL | 1 |
| 4 | NULL | 2 |
+---------+--------+--------+
So I want to stack ppp and ccc on top of each other, filling in NULL values where there isn't an entry in the other table. I have been experimenting with joins and haven't been able to produce this result. The closest I came was
SELECT
*
FROM
mmm,
ppp,
ccc
WHERE
mmm.main_id = ppp.main_id OR
mmm.main_id = ccc.main_id
GROUP BY
mmm.main_id
;
but that doesn't quite do it properly because there are still non-NULL values in the table with non-matching main_ids.

Answer 1

You want left joins:
select main_id, p.ppp_id, c.ccc_id
from mmm m
left join ppp p using (main_id)
left join ccc c using (main_id)

Answer 2

You can use CASE statements to replace values with NULL under certain conditions, and use DISTINCT instead of GROUP BY.
SELECT DISTINCT
mmm.main_id,
CASE
WHEN mmm.main_id = ppp.main_id THEN
ppp.ppp_id
ELSE
NULL
END AS ppp_id,
CASE
WHEN mmm.main_id = ccc.main_id THEN
ccc.ccc_id
ELSE
NULL
END AS ccc_id
FROM
mmm,
ppp,
ccc
WHERE
mmm.main_id = ppp.main_id OR
mmm.main_id = ccc.main_id
;