MySQL 8.0

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create table `offers` (
    fk_companies int,
    offers_fk_requests int
);
create table `companies` (
    companies_id int
);
create table `requests` (
    id int,
    fk_companies int
);
insert into `offers` values
    (1001,null),
    (1002,null),
    (1003,null),
    (1004,null),
    (1001,null),
    (1002,null),
    (1003,null),
    (1004,null),
    (null,2001),
    (null,2001),
    (null,2002),
    (null,2002),
    (null,2003),
    (null,2003),
    (null,2004),
    (null,2004);
insert into `companies` values
    (1001),
    (1002),
    (1003),
    (1004);
insert into `requests` values
    (2001,1001),
    (2002,1001),
    (2003,1002),
    (2004,1002);
SELECT *
FROM `offers`

LEFT JOIN `requests` ON `offers_fk_requests` = requests.`id`
JOIN `companies` AS companies1 ON requests.`fk_companies` = companies1.`companies_id`

LEFT JOIN `companies` AS companies2 ON offers.`fk_companies` = companies2.`companies_id`;


fk_companies
offers_fk_requests
id
fk_companies
companies_id
companies_id


null
2001
2001
1001
1001
null

null
2001
2001
1001
1001
null

null
2002
2002
1001
1001
null

null
2002
2002
1001
1001
null

null
2003
2003
1002
1002
null

null
2003
2003
1002
1002
null

null
2004
2004
1002
1002
null

null
2004
2004
1002
1002
null


SELECT *
FROM `offers`

LEFT JOIN `requests` ON `offers_fk_requests` = requests.`id`
JOIN `companies` AS companies1 ON requests.`fk_companies` = companies1.`companies_id`;


fk_companies
offers_fk_requests
id
fk_companies
companies_id


null
2001
2001
1001
1001

null
2001
2001
1001
1001

null
2002
2002
1001
1001

null
2002
2002
1001
1001

null
2003
2003
1002
1002

null
2003
2003
1002
1002

null
2004
2004
1002
1002

null
2004
2004
1002
1002


SELECT *
FROM `offers`

LEFT JOIN `companies` AS companies2 ON offers.`fk_companies` = companies2.`companies_id`;


fk_companies
offers_fk_requests
companies_id


1001
null
1001

1002
null
1002

1003
null
1003

1004
null
1004

1001
null
1001

1002
null
1002

1003
null
1003

1004
null
1004

null
2001
null

null
2001
null

null
2002
null

null
2002
null

null
2003
null

null
2003
null

null
2004
null

null
2004
null


SELECT *
FROM `offers`

LEFT JOIN `requests` ON `offers_fk_requests` = requests.`id`
JOIN `companies` AS companies1 ON requests.`fk_companies` = companies1.`companies_id`
UNION ALL SELECT null, null, null, `fk_companies`, `companies_id`
FROM `offers`

LEFT JOIN `companies` AS companies2 ON offers.`fk_companies` = companies2.`companies_id`
WHERE `companies_id` IS NOT NULL;


fk_companies
offers_fk_requests
id
fk_companies
companies_id


null
2001
2001
1001
1001

null
2001
2001
1001
1001

null
2002
2002
1001
1001

null
2002
2002
1001
1001

null
2003
2003
1002
1002

null
2003
2003
1002
1002

null
2004
2004
1002
1002

null
2004
2004
1002
1002

null
null
null
1001
1001

null
null
null
1002
1002

null
null
null
1003
1003

null
null
null
1004
1004

null
null
null
1001
1001

null
null
null
1002
1002

null
null
null
1003
1003

null
null
null
1004
1004


SELECT `requests`.`id` AS `requests_id`, coalesce(`requests`.`fk_companies`, `companies`.`companies_id`) AS `companies_id`
FROM `offers`

LEFT JOIN `companies` ON `offers`.`fk_companies` = `companies`.`companies_id`
LEFT JOIN `requests` ON `offers`.`offers_fk_requests` = `requests`.`id`;


requests_id
companies_id


null
1001

null
1002

null
1003

null
1004

null
1001

null
1002

null
1003

null
1004

2001
1001

2001
1001

2002
1001

2002
1001

2003
1002

2003
1002

2004
1002

2004
1002

fk_companies	offers_fk_requests	id	fk_companies	companies_id	companies_id
null	2001	2001	1001	1001	null
null	2001	2001	1001	1001	null
null	2002	2002	1001	1001	null
null	2002	2002	1001	1001	null
null	2003	2003	1002	1002	null
null	2003	2003	1002	1002	null
null	2004	2004	1002	1002	null
null	2004	2004	1002	1002	null