SQL - min(date) with conditionals - mysql

Question

We ran a promotion where users can receive their first subscription order free. Price = $0.00 when a user uses the promo. I am interested in the data from Example A.
Example A - User 50 started with the promo and continued for two months
order_id user_id price created_at
1 50 0.00 2018-01-15
5 50 20.00 2018-02-15
9 50 20.00 2018-03-15
Example B - User 100 was already an active subscriber who cancelled his account and reactivated with the promo, I do not wish to count him
order_id user_id price created_at
2 100 20.00 2018-01-16
3 100 0.00 2018-01-17
7 100 20.00 2018-02-17
--Here is my query--
This returns all users who have multiple orders
WHERE at least one of their orders has a price = 0.00
-This dataset returns example A and example B
--My question--
Most of this data is correct (Example A) but a handful of them I want to omit because they are skewing my data (Example B). I want to remove Example B users.
I want to remove people who's first order was not the promo.
How can I request that their FIRST order had a price = 0.00? I was thinking something with min(created_at)?

Answer 1

You can get the time of the first order using:
select user_id, min(created_at) as min_ca
from t
group by user_id;
Next, you can get the price of the first order using:
select oi.*
from order_items oi join
(select user_id, min(created_at) as min_ca
from order_items oi
group by user_id
) ooi
on oi.user_id = ooi.user_id and oi.created_at = ooi.min_ca
where oi.price = 0.00;
Then you can get all records, using join, in, or exists;
select oi.*
from order_items oi join
order_items oi1
on oi.user_id = oi1.user_id join
(select user_id, min(created_at) as min_ca
from order_items oi
group by user_id
) u1
on oi1.user_id = u1.user_id and oi1.created_at = u1.min_ca
where oi1.price = 0.00;

Answer 2

You can use EXISTS to check that for the record with zero price there is no earlier created_at:
SELECT COUNT(*), user_id
FROM Promo
WHERE user_id IN (
-- Query below yields [user_id]s of users who got the promo
-- that wasn't a result of a cancellation and re-activation
SELECT user_id
FROM Promo p
WHERE p.price = 0 AND NOT EXISTS (
-- Look for a record with the same user ID and an earlier date
-- than p.created_at, which is the date of the promo with 0.00 price
SELECT *
FROM Promo pp
WHERE pp.user_id=p.user_id AND pp.created_at < p.created_at
)
)
GROUP BY user_id