Postgresql remove values from foreign key that has a cyclic reference and also is referenced in a primary table - postgresql

Question

There are 2 tables:
the first one is the Father Table
create table win_folder_principal(
id_folder_principal serial primary key not null,
folder_name varchar(300)not null
);
and the table that has a cyclic reference
create table win_folder_dependency(
id_folder_dependency serial primary key not null,
id_folder_father int not null,
id_folder_son int not null,
foreign key(id_folder_father)references win_folder_principal(id_folder_principal),
foreign key(id_folder_son)references win_folder_principal(id_folder_principal)
);
however i found a very interesting situation, if i wanna remove a value from the table father that has a kid and that kid has more kids, is there any way to remove the values from the last to the first but also those values be removed from the Father table?
**WIN_FOLDER_PRINCIPAL**
| Id | Folder_Name|
| 23 | new2 |
| 24 | new3 |
| 13 | new0 |
| 22 | new1 |
| 12 | nFol |
And this are the value stored in the Win_Folder_Dependency
**WIN_FOLDER_DEPENDENCY**
| Id_Father | Id_Son |
| 12 | 13 |
| 13 | 22 |
| 22 | 23 |
| 23 | 24 |
and this is the query that i use to know the values in the dependency and principal table.
SELECT m2.id_folder_principal AS "Principal",
m.folder_name AS "Dependency",
m2.id_folder_principal AS id_principal,
m.id_folder_principal AS id_dependency
FROM ((win_folder_dependency md
JOIN win_folder_principal m ON ((m.id_folder_principal = md.id_folder_son)))
JOIN win_folder_principal m2 ON ((m2.id_folder_principal = md.id_folder_father)))
If i wanna remove the folder with the Id_Principal 13 i need to remove the other relations that exists in the Folder_Dependency table, but also remove the value from the Folder_Principal
is there any way to achieve that cyclic delete?

Answer 1

This anonymous code block will accumulate all the principles rooted with ID 13 searching down the dependency tree in an array parameter named l_Principles. It then deletes all the dependency records where either the father or son (or both) are contained in l_Principles, and then deletes all the principle records identified in l_Principles:
DO $$DECLARE
l_principles int[];
BEGIN
with recursive t1(root, child, pinciples) as (
select id_folder_father
, id_folder_son
, array[id_folder_father, id_folder_son]
from win_folder_dependency
where id_folder_father = 13
union all
select root
, id_folder_son
, pinciples||id_folder_son
from win_folder_dependency
join t1
on id_folder_father = child
and not id_folder_son = any(pinciples) -- Avoid cycles
)
select max(pinciples) into l_principles from t1 group by root;
delete from win_folder_dependency
where id_folder_father = any(l_principles)
or id_folder_son = any(l_principles);
delete from win_folder_principal
where id_folder_principal = any(l_principles);
end$$;
/
With your provided sample data, the end result will be only one record remaining in the win_folder_principal and no records in the win_folder_dependency table.

Answer 2

If you wan to delete a record from win_folder_principal you must first remove the references to it in win_folder_dependency like so:
delete from win_folder_dependency where 13 in (id_folder_father, id_folder_son);
before you delete the record from win_folder_principal like so:
delete from win_folder_principal where id_folder_principal = 13;
Alternatively if you build your second table like this:
create table win_folder_dependency(
id_folder_dependency serial primary key not null,
id_folder_father int not null,
id_folder_son int not null,
foreign key(id_folder_father)references win_folder_principal(id_folder_principal) on delete cascade,
foreign key(id_folder_son)references win_folder_principal(id_folder_principal) on delete cascade
);
Note the on delete cascade directives, then you can just delete from the principal table, and the references in the dependency table will be deleted as well.