How to get average with recursive query - sql

Question

I'm trying to write recursive query with postgres and its working fine and returning me all users which comes under user 5 with this codes :
WITH RECURSIVE subordinates AS (
SELECT
id,
supervisor_id,
name
FROM
employees
WHERE
id = 5
UNION
SELECT
e.id,
e.supervisor_id,
e.name
FROM
employees e
INNER JOIN subordinates s ON s.id = e.supervisor_id
) SELECT
*
FROM
subordinates;
But I've 1 more table in it which is called biscuits which have 3 columns type, weight and cooked_by_employee_id
now I want average weight of biscuits with id, name and supervisor_id but just small twist is if user id 1 comes under 3 and 3 comes under 4 and 4 comes under 5 then it should return average weight of all 1,3,4 and 5 cooked_by_employee_id records and 1 user can have multiple records in biscuits
I tried this but not working
WITH RECURSIVE subordinates AS (
SELECT
e.id,
e.supervisor_id,
e.name,
AVG(b.weight)
FROM
employees e
LEFT JOIN burrito b ON
e.id=b.cooked_by_employee_id
WHERE
e.id = 5
UNION
SELECT
e.id,
e.supervisor_id,
e.name,
b.weight
FROM
employees e
INNER JOIN subordinates s ON s.id = e.supervisor_id
LEFT JOIN burrito b ON b.cooked_by_employee_id=e.supervisor_id
) SELECT
*
FROM
subordinates;
Sample data :
employees :
+----+------+---------------+
| id | name | supervisor_id |
+----+------+---------------+
| 1 | a | 3 |
+----+------+---------------+
| 2 | b | 4 |
+----+------+---------------+
| 3 | c | 5 |
+----+------+---------------+
| 4 | d | 0 |
+----+------+---------------+
| 5 | e | 0 |
+----+------+---------------+
burrito :
+----+-------+--------+-----------------------+
| id | type | weight | cooked_by_employee_id |
+----+-------+--------+-----------------------+
| 1 | sweet | 1 | 1 |
+----+-------+--------+-----------------------+
| 2 | salty | 2 | 1 |
+----+-------+--------+-----------------------+
| 3 | sweet | 3 | 3 |
+----+-------+--------+-----------------------+
So if it runs for employee_id 5 then it should return
+-------------+------+-----------------------------------------------------+
| employee_id | name | weight |
+-------------+------+-----------------------------------------------------+
| 5 | e | 2 (Average 2 because both 1 and 3 id comes under 5) |
+-------------+------+-----------------------------------------------------+

Answer 1

You have the recursive CTE figured out and that could be half (or more) of the battle. The following however builds a slightly different one. The recursive cte (path_to_chef) gets employee ids and builds a path from the top level to the employee. The second cte (cook_chef) then strips the path string back to the top level employee id. The result then contains 2 columns, the employee id and the employee id for the root level employee. This then sets the stage to join with the other tables for the final result.
The query can work as stand alone and for a single root level by supplying id where indicated. But I tend to create generic reusable routines so I've wrapped it into a SQL function.
create or replace function chef_avg_burrito_weight()
returns table ( employee_id integer
, name text
, weight numeric
)
language sql
as $$
with recursive path_to_chef as
( select id, trim(to_char(id, '9999'))||'>' path
from employees
where supervisor_id = 0
union all
select e.id, path || trim(to_char(e.id, '9999'))||'>'
from employees e
join path_to_chef c on e.supervisor_id = c.id
) -- select * from path_to_chef
, cook_chef as
(select id cook
, substring(path from '^([[:digit:]]+)>')::integer chef
from path_to_chef
) -- select * from cook_chef
select e.id, e.name, round(avg(b.weight),2) average_weight
from burrito b
join cook_chef c on (b.cooked_by_employee_id = c.cook)
join employees e on (e.id = c.chef)
-- where chef = 5
group by e.id, e.name;
$$;
-- test
select *
from chef_avg_burrito_weight()
where employee_id = 5;