How to show difference between two tables?
MySQL knows nothing about FULL OUTER JOIN, so

SELECT t0.param, t1.value, t2.value
FROM (SELECT param from db1.settings
          UNION
      SELECT param from db2.settings) t0
LEFT JOIN db1.settings t1 ON t0.param = t1.param
LEFT JOIN db2.settings t2 ON t0.param = t2.param
-- remove duplicates if not need in
WHERE NOT (t1.value &lt;=&gt; t2.value)