Recursive delete of multiple rows in one table - sql

Question

I have a problem with my postgres database. I have a table Tasks with 3 columns: ID, Name and Parent_ID (which refers to another task id in this table):
id | name | parent_id
---+------+-----------
1 | A | 0
2 | B | 1
3 | C | 2
4 | D | 1
5 | E | 0
6 | F | 0
So basically it's like this:
1. A
2. B
3. C
4. D
5. E
6. F
What I'm trying to do is to delete task A, and delete all of its children and all children of children etc etc..(in this case B and D, along with C as its children of B which is deleted) something like cascade delete, but i cant do this. Maybe any function will work?
The result after delete should be
id | name | parent_id
---+------+-----------
5 | E | 0
6 | F | 0
Hope you guys can help me.

Answer 1

You only need a cascading FK-constraint:
\i tmp.sql
CREATE TABLE employees (
id serial PRIMARY KEY
, name VARCHAR NOT NULL
, parent_id INT REFERENCES employees(id) ON DELETE CASCADE
);
INSERT INTO employees
VALUES (1,'A', NULL),(2,'B', 1),(3,'C', 2),
(4,'D', 1),(5,'E', NULL),(6,'F', NULL);
DELETE FROM employees
WHERE name = 'A'
;
SELECT * FROM employees
;
----------
Result:
----------
DROP SCHEMA
CREATE SCHEMA
SET
CREATE TABLE
INSERT 0 6
DELETE 1
id | name | parent_id
----+------+-----------
5 | E |
6 | F |
(2 rows)

Answer 2

You can build the list of all descendent rows from the starting name with a recursive query, and then use it as a filter for the deletetions.
with recursive cte(id, parent_id) as (
select id, parent_id from mytable where name = 'A'
union all
select t.id, t.parent_id from mytable t inner join cte c on c.id = t.parent_id
)
delete from mytable where id in (select id from cte)
Demo on DB Fiddle - table content after executing the query:
id | name | parent_id
-: | :--- | --------:
5 | E | 0
6 | F | 0

Answer 3

Use recursive cte to get all rows with Name = 'A' and it's subordinates.
Then delete it from table employees.
Here is the step to create the table:
Sample table: CREATE TABLE employees (
id serial PRIMARY KEY,
name VARCHAR NOT NULL,
parent_id INT
);
INSERT INTO employees
VALUES (1,'A', 0),(2,'B', 1),(3,'C', 2),
(4,'D', 1),(5,'E', 0),(6,'F', 0);
Query:
WITH RECURSIVE subordinates AS (
SELECT
id,
parent_id,
name
FROM
employees
WHERE
name = 'A'
UNION
SELECT
e.id,
e.parent_id,
e.name
FROM
employees e
INNER JOIN subordinates s ON s.id = e.parent_id
)
DELETE
FROM employees
WHERE id in (
SELECT
id
FROM
subordinates);
SELECT * FROM employees;