Select total average of averages grouped by id - sql

Question

In my database that represents a car service station, I am trying to figure out a SQL query that would give me a total average of how much does the customer pays for a single service but instead of getting AVG() of the price on all existing Invoices, I want to group the invoices by the same reservation_id. After that, I would like to get the total average of all of those grouped results.
I am using the two tables listed in the picture below. I want to get the value of a total average price by applying AVG() on all averages that are made by grouping prices by the same FK Reservation_reservation_id.
I tried to make this into a single query but I failed so I came looking for help from more experienced users. Also, I need to select (get) only the result of the total average. This result should give me an overview of how much each customer pays on average for one reservation.
Thanks for your time

Answer 1

You appear to want to aggregate twice:
SELECT AVG( avg_price ) avg_avg_price
FROM (
SELECT AVG( price ) AS avg_price
FROM invoice
GROUP BY reservation_reservation_id
)
Which, for the sample data:
CREATE TABLE invoice ( reservation_reservation_id, price ) AS
SELECT 1, 10 FROM DUAL UNION ALL
SELECT 1, 12 FROM DUAL UNION ALL
SELECT 1, 14 FROM DUAL UNION ALL
SELECT 1, 16 FROM DUAL UNION ALL
SELECT 2, 10 FROM DUAL UNION ALL
SELECT 2, 11 FROM DUAL UNION ALL
SELECT 2, 12 FROM DUAL;
Outputs:
AVG_AVG_PRICE
12
db<>fiddle here

Answer 2

If you want this per customer:
SELECT customer_customer_id, AVG(avg_reservation_price)
FROM (SELECT i.customer_customer_id, i.reservation_reservation_id,
AVG(i.price) as avg_reservation_price
FROM invoice i
GROUP BY i.customer_customer_id, i.reservation_reservation_id
) ir
GROUP BY customer_customer_id;
If you want this for a particular "checkout reason" -- which is the closest that I imagine that "service" means -- then join in the reservations table and filter:
SELECT customer_customer_id, AVG(avg_reservation_price)
FROM (SELECT i.customer_customer_id, i.reservation_reservation_id,
AVG(i.price) as avg_reservation_price
FROM invoice i JOIN
reservation r
ON i.reservation_reservation_id = r.reservation_id
WHERE r.checkup_type = ?
GROUP BY i.customer_customer_id, i.reservation_reservation_id
) ir
GROUP BY customer_customer_id;

Answer 3

You might want to try the below:
with aux (gr, subgr, val) as (
select 'a', 'a1', 1 from dual union all
select 'a', 'a2', 2 from dual union all
select 'a', 'a3', 3 from dual union all
select 'a', 'a4', 4 from dual union all
select 'b', 'b1', 5 from dual union all
select 'b', 'b2', 6 from dual union all
select 'b', 'b3', 7 from dual union all
select 'b', 'b4', 8 from dual)
SELECT
gr,
avg(val) average_gr,
avg(avg(val)) over () average_total
FROM
aux
group by gr;
Which, applied to your table, would result in:
SELECT
reservation_id,
avg(price) average_rn,
avg(avg(price)) over () average_total
FROM
invoices
group by reservation_id;