Since there is no SQL editor available, I wonder how I can create a metric with join table. Am I doing it wrong?
You can access an SQL editor in the questions. Have a read here:
You can “kind of” do joins with embedded questions:
maybe that helps also
You can also “hard” join tables within the admin section --> Datamodel. You can there connect fields from different tables (foreign key - entity key). Then you can use fields from both tables in the query builder.
Although I would recommend using SQL for almost everything, as you can flexible join different fields only in SQL and you have by far more options in SQL anyway (like renaming columns, calculating new metrics and so on).