When a stream query operates over two input streams it is referred to as a join operation. There are two forms of join operation available in EPL. Each form takes two input streams and produces a single output stream of combined items. A cross-join joins every event from one stream's window with every event in the other stream's window. An equi-join joins events only when they have matching keys.

Join operations, particularly cross-joins, can create many more output events than input events, not just the same or fewer.