Works basically as a normal Join operator with one exception: elements on the left input will be transfered even if there is no join partner. In this case the schema will be filled with null values.
Parameter
See Join operator.
Example
PQL
bidWnd = ELEMENTWINDOW({SIZE = 1}, bid) auctionWnd = ELEMENTWINDOW({SIZE = 1}, auction) out = LEFTJOIN({predicate='id=auction'}, auctionWnd, bidWnd)