Advanced SQL challenges

Contents:

• Subscription price changes
• Apportioning payments

Subscription price changes

Background

At my seltzer subscription box startup, customers get billed once a month. In between billing dates, customers can upgrade or downgrade their subscription box (which impacts how many seltzers they'll be sent).

These upgrade and downgrades are captured in a table, named subscription_price_changes:

select * from `advanced-sql-challenges`.`subscription_price_changes`.`subscription_price_changes`

change_id	subscription_id	price	changed_at
1	1	50	2020-01-10
2	1	60	2020-01-15
3	2	40	2020-01-12
...	...	...	...

However, the change only takes effect on the date the customer gets rebilled — as above, this only happens once a month (the day of the month varies per customer).

Fortunately, I also have a table of rebillings, with one record per billing date.

select * from `advanced-sql-challenges`.`subscription_price_changes`.`rebillings`

rebilling_id	subscription_id	rebilled_at
1	1	2020-02-01
2	1	2020-03-01
3	3	2020-02-12

I need to produce a table that tells me the effective dates of each subscription price change:

select * from `advanced-sql-challenges`.`subscription_price_changes`.`effective_subscription_changes`

subscription_id	new_price	changed_at	effective_at
1	60	2020-01-15	2020-02-01

Things worth noting in the final table.

• We don't care about change_id = 1 from the original table — that's because it was superseded by change_id = 2 before it took effect.
• We also haven't included change_id = 3 here, as it's associated with subscription_id = 2, which doesn't have an associated rebill (perhaps the subscription is actually cancelled, and the change was needlessly applied).
• The effective_at date comes from the rebillings table

Instructions

1. Write the SQL to produce the effective_subscription_changes table.
2. Bonus: Write SQL to prove that your effective_subscription_changes table matches the sample one

Note: the above tables can all be queried from BigQuery — the dbt tutorial has a handy section on how to create a BigQuery account and query a public dataset.

I recommend that you use the public BigQuery tables to perform this task. You may also load the CSVs in the subscription-price-changes directory of this repo into the warehouse of your choosing.

Apportioning payments

A task marketplace startup is grappling with how to apportion their payments for accounting purposes.

On the platform:

• A user posts a task
• A worker offers to do it for a particular price
• A user accepts the offer, and the amount goes into escrow
• Upon successful completion of the task, the worker receives all the money¹

The associated payments for a typical task (task_id = 1) in the database look like this:

payment_id	task_id	payment_type	amount
1	1	inbound	50
2	1	payout	50

For accounting purposes, it needs to be transformed to this:

task_id	inbound_payment_id	inbound_amount	payout_amount
1	1	50	50

Seems do-able.

However, sometimes after the money is placed in escrow, the task gets cancelled, and all money is refunded to the user.

The associated payments for such a task (task_id = 2) looks like this:

payment_id	task_id	payment_type	amount
3	2	inbound	30
4	2	refund	30

And for accounting purposes it needs to looks like this:

task_id	inbound_payment_id	inbound_amount	payout_amount	refund_amount
2	3	30	0	30

Also seems do-able.

However, sometimes after the money is placed in escrow, a second inbound payment occurs because the task was bigger than expected. Assuming that the worker is paid out fully, the payments might look like this for such a task (task_id = 3):

payment_id	task_id	payment_type	amount
5	3	inbound	40
6	3	inbound	20
7	3	payout	60

And for accounting purposes it needs to looks like this:

task_id	inbound_payment_id	inbound_amount	payout_amount	refund_amount
3	5	40	40	0
3	6	20	20	0

Note: things are getting complicated here. For complicated accounting reasons required by the finance department, it's important that the payout payment is apportioned over each inbound payment such that the payout amount does not exceed the inbound amount — here is was split into $40 and $20 to match that requirement. Okay, getting tricky

HOWEVER, sometimes a partial refund needs to occur for a task — maybe only half the task was done. And every so often that occurs on a task that had two inbound payments.

The associated payments for such a task (task_id in (4, 5)) looks like this:

payment_id	task_id	payment_type	amount
8	4	inbound	20
9	4	payout	15
10	4	refund	5
11	5	inbound	20
12	5	inbound	40
13	5	inbound	30
14	5	payout	25
15	5	refund	65

And for accounting purposes it looks like this:

task_id	inbound_payment_id	inbound_amount	payout_amount	refund_amount
4	8	20	15	5
5	11	20	20	0
5	12	40	5	35
5	13	30	0	30

Y I K E S, that task_id = 5 is particularly nasty — there's three inbound payments, and we need to split the payout payment across the first two inbound payments, and then split the refund across the second and third payments. And no, apparently we can't apportion them according to their ratios — we instead need to first fully "refund" the first inbound payment (inbound_payment_id = 11), and then partially refund the second inbound payment (inbound_payment_id = 12)).

Exercise

1. Write the SQL to transform the ledger of payments into the format required by accounting. Use the following BigQuery tables, or load the CSVs in apportioning-payments into the warehouse of your choice:

-- input data
select * from `advanced-sql-challenges`.`apportioning_payments`.`payments`

-- sample output for comparison
select * from `advanced-sql-challenges`.`apportioning_payments`.`inbound_payment_states`

2. List the assumptions that you are making about your source data as you write this

Bonus exercises

1. Write SQL to prove that your inbound_payment_states table matches the sample result.
2. Consider how this might work for in-progress tasks that only have inbound payments, where an additional column in_escrow tracks any money held in the bank account.
3. Consider what might happen if there are two refund payments for the one task

¹Perhaps their accounting problems are the least of their woes since currently the business does not capture any fees.