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SELECT *
FROM catalog_product_entity AS PRO
INNER JOIN catalog_product_entity_media_gallery AS MED ON PRO.entity_id = MED.entity_id
INNER JOIN catalog_product_entity_media_gallery_value AS MEDV ON MED.value_id = MEDV.value_id
WHERE PRO.type_id = "configurable" AND PRO.entity_id = MED.entity_id AND MEDV.disabled = 1 AND MEDV.position = 0;

//UPDATE 
UPDATE catalog_product_entity_media_gallery_value AS MEDV
INNER JOIN catalog_product_entity_media_gallery AS MED ON MED.value_id = MEDV.value_id
INNER JOIN catalog_product_entity AS PRO ON PRO.entity_id = MED.entity_id 
SET MEDV.disabled = 0
WHERE PRO.type_id = "configurable" AND PRO.entity_id = MED.entity_id AND MEDV.disabled = 1 AND MEDV.position = 0;