SELECT * FROM categories
LEFT JOIN subscriptions
ON subscriptions.category_id = categories.category_id
WHERE
 IF EXISTS(subscriptions.is_enable)
  categories.type = 'premium' AND
  subscriptions.is_enable = 1
ELSE
  categories.type = 'premium' AND

SELECT * FROM categories
LEFT JOIN subscriptions
ON subscriptions.category_id = categories.category_id
WHERE
  (EXISTS(subscriptions.is_enable) AND
  categories.type = 'premium' AND
  subscriptions.is_enable = 1)
OR
  (categories.type = 'premium' AND...)

SELECT * FROM categories
LEFT JOIN subscriptions
ON subscriptions.category_id = categories.category_id
WHERE (subscriptions.is_enable IS NULL OR subscriptions.is_enable = 1)
  AND categories.type = 'premium'