Anti-Join in MongoDB

SQL databases implement anti-joins—finding rows with no match—through NOT EXISTS. Although anti-join is not a distinct relational algebra operator, it can be derived from set difference and semi-join. A naive implementation that scans both sets to find the complement would not scale, so databases use short-circuit evaluation: the inner scan stops as soon as a match is found or ruled out, because only existence matters. That is why a subquery without any JOIN keyword can still appear as an Anti J