Use the result of a sub-query outside of the sub-query

Question

I have a table structured like this.

User_id	Subscription_type	timestamp
100	PAYING	2/10/2021
99	TRIAL	2/10/2021
100	TRIAL	15/9/2021

I want my output to be the same, with an additional column pulling the trial start date when the subscriber converts to a paying subscription.

User_id	Subscription_type	timestamp	Trial_Start_date
100	PAYING	2/10/2021	15/9/2021
99	TRIAL	2/10/2021
100	TRIAL	2/10/2021

At the moment, I have this query:

SELECT *,
CASE WHEN    
            (SELECT `subscription_type` FROM subscription_event se1
            WHERE se1.`timestamp` < se.`timestamp` AND se1.user_id = se.user_id
            ORDER BY user_id DESC LIMIT 1) = 'TRIAL'
        then se1.`timestamp` else 0 end as "Converted_from_TRIAL"

FROM subscription_event se

I have an error message with se1. timestamp not been defined. I understand why, but I cannot see a workaround.

Any pointer?

Answer 1

If you need to get two values out of the subquery, you have to join with it, not use it as an expression.

SELECT se.*,
    MAX(se1.timestamp) AS Converted_from_TRIAL
FROM subscription_event AS se
LEFT JOIN subscription_event AS se1 ON se.user_id = se1.user_id AND se1.timestamp < se.timestamp AND se1.subscription_type = 'TRIAL'
GROUP BY se.user_id, se.subscription_type, se.timestamp

Answer 2

Thanks a lot. For some reasons I needed to declare explicitely in SELECT the variables used in the GROUP BY. Not sure why ( I am using MySQL5.7 so maybe it is linked with that), In any case. this is the working query.

SELECT se.user_id, se.subscription_type, se.timestamp,
    MAX(se1.timestamp) AS Converted_from_TRIAL
    
FROM subscription_event AS se

LEFT JOIN subscription_event AS se1 ON se.user_id = se1.user_id AND se1.timestamp < se.timestamp AND se1.subscription_type = 'TRIAL'

GROUP BY se.user_id, se.subscription_type, se.timestamp