SQL - select data from various tables based on multiple values in CELL - mysql

Question

I'm working with an old database and I want to select data spread across different tables but in an recurring manner based on multiple values in one cell.
There are three tables (here with fictional data): treaties, parties, related_parties. The treaties table includes information about specific treaties as well as the ID of the parties that signed it. The parties table comprises information about the parties, whereas the related_parties table consists of information about parties that were related (the related_partner_id being the IDs used the parties table).
Table 1: treaties
id
name
party_id
1
Peace of Westphalia
49, 80
2
Peace of Rijswijk
49, 50, 81
Table 2: parties
party_id
party_name
49
Holy Roman Empire
50
Dutch Republic
51
Mainz
52
Cologne
80
France
81
Sweden
82
Paris
83
Bordeaux
Table 3: related_parties
party_id
related_party_id
49
51, 52
80
82, 83
What I want as the output is something like this, where information about the parties is gathered for every value in the relevant cells. So for the first treaty (1) this would be:
id
name
party_id
party_name
related_party_id
related_party_name
1
Peace of Westphalia
49
Holy Roman Empire
51
Mainz
1
Peace of Westphalia
49
Holy Roman Empire
52
Cologne
1
Peace of Westphalia
80
France
82
Paris
1
Peace of Westphalia
80
France
83
Bordeaux
Is this at all doable? So far every query I've created only retrieves the data pertaining to the first value in a cell.

Answer 1

To achieve this, the first thing to do would be divide columns into multiple rows. You have to define the maximum amount of divisions your column can have, but i came with this query which becomes pretty handy for this task:
https://www.appsloveworld.com/mysql/100/79/split-string-into-multiple-rows-in-sql
Applied to this particular case:
select t5.*,t6.party_name as related_party_name from (
select t3.id,t3.name,t3.party_id, t4.party_name,related_party_id
from
(
select t2.id, t2.name, t1.party_id,t1.related_party_id from
(
select * from (
select
party_id,(SUBSTRING_INDEX(SUBSTRING_INDEX(related_party_id, ',', NS.n), ',', -1)) as related_party_id
from (
select 1 as n union all
select 2 union all
select 3 union all
select 4 union all
select 5 union all
select 6 union all
select 7 union all
select 8 union all
select 9 union all
select 10
) NS
inner join related_parties B ON NS.n <= CHAR_LENGTH(B.related_party_id) - CHAR_LENGTH(REPLACE(B.related_party_id, ',', '')) + 1
) divided_related_parties
)t1
left join
(
select * from (
select
id,name,(SUBSTRING_INDEX(SUBSTRING_INDEX(party_id, ',', NS.n), ',', -1)) as party_id
from (
select 1 as n union all
select 2 union all
select 3 union all
select 4 union all
select 5 union all
select 6 union all
select 7 union all
select 8 union all
select 9 union all
select 10
) NS
inner join treaties B ON NS.n <= CHAR_LENGTH(B.party_id) - CHAR_LENGTH(REPLACE(B.party_id, ',', '')) + 1
) divided_treaties
)t2
using(party_id)
)t3
left join
(
select * from parties
)t4
using(party_id)
)t5
left join
(
select * from parties
)t6
on(t5.related_party_id=t6.party_id)
order by 1,3
This is exactly what you want (add more elements to union all if you expect more than 10 parties in a treaty).