How can I pass on a value in the hierarchy based on the value of the parent using an hierarchial query?
with a (id, parent_id, name, user_id, discount, lvl) as
(
select
c.id, c.parent_id, c.name, cd.user_id, cd.discount, 1 as lvl from category c
LEFT JOIN category_discount cd
ON c.id = cd.category_id AND cd.user_id = 1
where parent_id is null
union all
select
c2.id, c2.parent_id, c2.name, cd.user_id,
nvl(cd.discount, a.discount),
lvl + 1
from category c2
LEFT JOIN category_discount cd
ON c2.id = cd.category_id AND cd.user_id = 1
join a on c2.parent_id = a.id
)
search depth first by id set dummy
select id, parent_id, name, user_id, discount, lvl from a;
ID PARENT_ID NAME USER_ID DISCOUNT LVL
---------- ---------- ---- ---------- ---------- ----------
1 root 1 30 1
2 1 C1 30 2
3 1 C3 30 2
4 1 C4 1 20 2
5 4 C5 20 3
6 4 C6 20 3
9 6 C9 20 4
10 6 C10 20 4
7 1 C7 1 25 2
11 7 C11 25 3
12 7 C12 25 3
Answer to original question:
Assuming the following based on your example data (but I am not sure this is what you want):
There is a discount for the whole branch defined by the root - I call this root_discount.
If a child (c1) has discount set, then that is the discount for that child.
If c2 is a child of c1, and c2 does not have discount set, then c2 inherits the discount of c1.
If c3 is a child of c2, and c3 does not have discount set, then c3 inherits the discount from the root (root_discount). This makes the discount 30 for rows with ID 9,10. Without this, it gets a lot simpler.
with a (id, parent_id, name, user_id, discount, prev_discount, root_discount, lvl) as
(
select
c.id, c.parent_id, c.name, cd.user_id, cd.discount, 1 as prev_discount,
discount as root_discount, 1 as lvl
from category c
LEFT JOIN category_discount cd
ON c.id = cd.category_id AND cd.user_id = 1
where parent_id is null
union all
select
c2.id, c2.parent_id, c2.name, cd.user_id,
nvl(cd.discount, case when prev_discount = 1 then a.discount else root_discount end),
case when cd.discount is not null then 1 else a.prev_discount - 1 end as prev_discount,
root_discount, lvl + 1
from category c2
LEFT JOIN category_discount cd
ON c2.id = cd.category_id AND cd.user_id = 1
join a on c2.parent_id = a.id
)
search depth first by id set dummy
select id, parent_id, name, user_id, discount, lvl from a;
ID PARENT_ID NAME USER_ID DISCOUNT LVL
---------- ---------- ---- ---------- ---------- ----------
1 root 1 30 1
2 1 C1 30 2
3 1 C3 30 2
4 1 C4 1 20 2
5 4 C5 20 3
6 4 C6 20 3
9 6 C9 30 4
10 6 C10 30 4
7 1 C7 1 25 2
11 7 C11 25 3
12 7 C12 25 3
I've used a recursive solution, replacing discount NULL values in child rows by the value of the parent row.
WITH x (id, parent_id, name, user_id, discount)
AS
(
SELECT
c.id, c.parent_id, c.name, cd.user_id, cd.discount
FROM
category c
LEFT JOIN
category_discount cd
ON c.id = cd.category_id
AND cd.user_id = 1
WHERE
c.parent_id IS NULL
UNION ALL
SELECT
c.id, c.parent_id, c.name, cd.user_id,
CASE WHEN cd.discount IS NULL THEN x.discount ELSE cd.discount END discount
FROM
x
JOIN
category c
ON c.parent_id = x.id
LEFT JOIN
category_discount cd
ON c.id = cd.category_id
AND cd.user_id = 1
)
SELECT
id,
parent_id,
name,
user_id,
discount
FROM
x;
ID | PARENT_ID | NAME | USER_ID | DISCOUNT -: | --------: | :--- | ------: | -------: 1 | null | root | 1 | 30 2 | 1 | C1 | null | 30 3 | 1 | C3 | null | 30 4 | 1 | C4 | 1 | 20 7 | 1 | C7 | 1 | 25 11 | 7 | C11 | null | 25 12 | 7 | C12 | null | 25 5 | 4 | C5 | null | 20 6 | 4 | C6 | null | 20 9 | 6 | C9 | null | 20 10 | 6 | C10 | null | 20
db<>fiddle here
You can do this without using recursive subquery factoring, with the magic of SYS_CONNECT_BY_PATH:
SELECT category.*, category_discount.user_id, category_discount.discount, sys_connect_by_path(category_discount.discount,':') discount_path, regexp_substr(rtrim(sys_connect_by_path(category_discount.discount,':'),':'),'[0-9]+$') actual_discount, LEVEL FROM category LEFT JOIN category_discount ON category.id = category_discount.category_id AND category_discount.user_id = 1 START WITH parent_id is null CONNECT BY PRIOR id = parent_idID | PARENT_ID | NAME | USER_ID | DISCOUNT | DISCOUNT_PATH | ACTUAL_DISCOUNT | LEVEL -: | --------: | :--- | ------: | -------: | :------------ | :-------------- | ----: 1 | null | root | 1 | 30 | :30 | 30 | 1 2 | 1 | C1 | null | null | :30: | 30 | 2 3 | 1 | C3 | null | null | :30: | 30 | 2 4 | 1 | C4 | 1 | 20 | :30:20 | 20 | 2 5 | 4 | C5 | null | null | :30:20: | 20 | 3 6 | 4 | C6 | null | null | :30:20: | 20 | 3 9 | 6 | C9 | null | null | :30:20:: | 20 | 4 10 | 6 | C10 | null | null | :30:20:: | 20 | 4 7 | 1 | C7 | 1 | 25 | :30:25 | 25 | 2 11 | 7 | C11 | null | null | :30:25: | 25 | 3 12 | 7 | C12 | null | null | :30:25: | 25 | 3
db<>fiddle here
The DISCOUNT and DISCOUNT_PATH columns in the output are only included to make it clearer how this works — ACTUAL_DISCOUNT is the one you need.