The window join requires the join on condition contains window starts equality of input tables and window ends equality of input tables. The semantic of window join is the same to the DataStream window join.
In the future, we can also simplify the join on clause to only include the window start equality if the windowing TVF is TUMBLE or HOP . Currently, the windowing TVFs must be the same of left and right inputs. This can be extended in the future, for example, tumbling windows join sliding windows with the same window size.