When writing a query in X++, the only available joins are INNER, OUTER and EXIST.
The inner join is translated to a CROSS JOIN, with (optionally) a WHERE clause.
In SQL we have the best practice to write relations between tables in the ON clause.
This because this is executed more early in the execution plan.

So use "ON" clause for the chosen relation between two tables,
then use the "WHERE" clauses to do additional filtering.

The "ON" clause is however not available from X++.
Over all system performance could benefit when this is introduced.