SQL - min(date) with conditionals

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