Combine all rows in a column where most rows are null - sql

Question

I am writing a query trying to match true account IDs to incorrect account IDs across two tables using the following query:
SELECT DISTINCT
p.visitor_id,
CASE WHEN p.visitor_id = c.username then accountId else null end as correct_account_id,
CASE WHEN c.accountId is null then p.account_id else null end as incorrect_account_id
FROM `a_table` p
LEFT JOIN `another_table` c
ON p.account_id = c.accountID
and am getting this result (single vistor_id subset):
visitor_id
correct_account_id
incorrect_account_id
1
null
id
1
id
null
1
null
null
I would like to create one row per visitor_id where there are no null values and just the two ids are listed.

Answer 1

It sounds like you want:
SELECT p.visitor_id,
MAX(CASE WHEN p.visitor_id = c.username then accountId else null end) as correct_account_id,
MAX(CASE WHEN c.accountId is null then p.account_id else null end) as incorrect_account_id
FROM a_table p LEFT JOIN another_table c ON p.account_id = c.accountID
GROUP BY p.visitor_id
I removed the distinct and added the group by. Group by p.visitor_id means you want one row per visitor_id.
I wrapped your two CASE statements in MAX which means we want the maximum value (within the visitor_id grouping) there. Depending on your DB, you might need MIN instead (some DBs order nulls after other values by default, some order nulls after other values by default). Either way, the intention here is to find a non-null value for the column.
The big assumption here is that for each visitor_id, you have at most one correct_account_id and at most one incorrect_account_id that you care about. If you have more than one for the same visitor_id, this will only get one of them (the max/min). (Given that you explicitly say you want one row per visitor_id, this seems like a safe assumption.)