Can't concat values with GROUP_CONCAT on joins?

The left join is useless. Because you have a condition on subscriber_data in the WHERE clause, that subquery will not return those rows for which there is no matching subscriber_data, so it effectively works as if you used INNER JOIN. You should add that condition to the left join condition, but it is impossible in this query layout.

Values from the outer query are not allowed in join conditions in the inner query.

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