Conditional subquery in where clause - mysql

Question

I'm trying to create a query with conditional logic where I only calculate revenue for the most recent records by each month using a datetime column (start_date), but only if there are multiple records in that month from the same account_id.
Here's a basic example of the schema after I join two tables (full schema in sqlfiddle link).
| account_id | plan_id | start_date | plan_interval | price |
|------------|---------|----------------------|---------------|-------|
| 1 | 1 | 2018-01-03T14:52:13Z | month | 39 |
| 1 | 3 | 2018-02-07T11:10:17Z | year | 999 |
| 1 | 2 | 2018-02-07T11:11:17Z | month | 99 |
In the above example, I would only like to include rows 1 and 3 in my output, as it's the one record from account_id 1 in January and the most recent of two records for account_id 1 in February.
SELECT
MONTH(start_date) AS month,
SUM(CASE WHEN plan_interval = 'month'
THEN price * .01
ELSE (price * .01)/12 END) AS mrr
FROM subscriptions
JOIN plans
ON plans.id = subscriptions.plan_id
WHERE Year(start_date) = 2018 AND
CASE WHEN (account_id = account_id
AND MONTH(start_date) = MONTH(start_date))
THEN (SELECT MAX(start_date) FROM subscriptions)
ELSE (SELECT start_date FROM subscriptions)
END
GROUP BY month
ORDER BY month ASC;
The case statement in the subquery above does not seem to work in doing this. It returns the data without filtering out records when the first condition is met.
Here is an example: sqlfiddle

Answer 1

This query returns the rows that you are asking for in the question:
SELECT s.*, p.plan_interval, p.price,
(CASE WHEN p.plan_interval = 'month'
THEN p.price * 0.01
ELSE (p.price * 0.01)/12
END) AS mrr
FROM subscriptions s JOIN
plans p
ON p.id = s.plan_id
WHERE YEAR(s.start_date) = 2018 AND
s.start_date = (SELECT MAX(s2.start_date)
FROM subscriptions s2
WHERE s2.account_id = s.account_id AND
EXTRACT(YEAR_MONTH FROM s2.start_date) = EXTRACT(YEAR_MONTH FROM s.start_date)
)
ORDER BY s.start_date ASC;
This uses a subquery to get the most recent record for a subscription for each month.
You can then aggregate this however you wish.
Notes about the query:
Table aliases make the query easier to write and to read.
The subquery uses the handy YEAR_MONTH option of EXTRACT(), so it handles both years and months.
For numeric constants between -1 and 1, I always prepend with a 0, so 0.12 rather than .12. If find that this makes the decimal point more obvious.

Answer 2

First work out the last entry by account and month (sub query a) join to subscriptions to get the plan_id and then get the plan
SELECT S.ACCOUNT_id,s.plan_id,s.start_date,p.Price,p.plan_interval,
case when p.plan_interval = 'month' then p.price * .01 /12 else p.price * .01 end as rev
from subscriptions s
join (select s.account_id,month(s.start_date), max(s.start_date) start_date
from subscriptions s
group by account_id,month(start_date)) a on a.account_id = s.account_id and a.start_date = s.start_date
join plans p on p.id = s.plan_id;
+------------+---------+---------------------+----------+---------------+--------------+
| ACCOUNT_id | plan_id | start_date | Price | plan_interval | rev |
+------------+---------+---------------------+----------+---------------+--------------+
| 1 | 1 | 2018-01-03 14:52:13 | 3900.00 | month | 3.25000000 |
| 1 | 2 | 2018-02-07 11:11:17 | 9900.00 | month | 8.25000000 |
| 2 | 3 | 2018-01-03 17:40:05 | 99900.00 | year | 999.00000000 |
+------------+---------+---------------------+----------+---------------+--------------+

Answer 3

In your case, the WHERE statement does not work because the CASE statement will always return a boolean.
CASE WHEN (account_id = account_id
AND MONTH(start_date) = MONTH(start_date))
THEN (SELECT MAX(start_date) FROM subscriptions)
ELSE (SELECT start_date FROM subscriptions)
END
Another approach to what you are building would involve using a subquery to order the columns the way you want within the groups.
SELECT
account_id,
month,
CASE WHEN plan_interval = 'month'
THEN price * .01
ELSE (price * .01)/12
END AS mrr
FROM (
SELECT *, MONTH(start_date) AS month
FROM subscriptions
INNER JOIN plans ON plans.id = subscriptions.plan_id
ORDER BY account_id, start_date DESC
) sq
GROUP BY account_id, month
This works because selecting columns in a GROUP BY will automatically take the first row that is returned by the subquery for a given group of columns.