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SELECT m.customer_id,
       smm.mid
FROM merchant m
JOIN sponsor_merchant_mid smm ON smm.merchant_id = m.id
WHERE m.customer_id IN (1)
UNION ALL
SELECT m.customer_id,
       rsmm.mid
FROM merchant m
JOIN reporting_sponsor_merchant_mid rsmm ON rsmm.merchant_id = m.id
WHERE m.customer_id IN (1);