I'm trying to use a FULL OUTER join in a recipe between 2 MySQL datasets. The option is not available while it appears that it should: "Not available in this mode, use a SQL or Hive database to use FULL OUTER joins."
MySQL does not officially support FULL OUTER joins.
There is common hack to emulate a FULL OUTER join with a left and a right join (as suggested in the comments of the documentation). But this hack is not implemented in the join (visual) recipe. I'm not sure it will be.
However, you could do it manually with the SQL (code) recipe.
I hope that helps,