MySQL WHERE if NULL select another column
Generally speaking you can use COALESCE

SELECT * 
FROM TABLE 
WHERE COALESCE(date_active,date_created) = 'value'


However this is not sargable. Depending on indexes available you may find the following

SELECT * 
FROM TABLE 
WHERE date_active = 'value' OR 
      (date_active IS NULL AND date_created = 'value')


Or even

SELECT * 
FROM TABLE 
WHERE date_active = 'value'
UNION ALL
SELECT * 
FROM TABLE 
WHERE  (date_active IS NULL AND date_created = 'value')


Performs better