Database
db={"table1":[{"url":"www.example1.com","name":"hans","age":30},{"url":"www.example2.com","name":"x","age":34},],"table2":[{"url":"www.example3.com","name":"y","age":35},{"url":"www.example4.com","name":"z","age":36},{"url":"www.example5.com","name":"b","age":37}]}
Query
db.table1.aggregate([{$match:{"url":{"$eq":"www.example3.com"}}},{"$unionWith":{"coll":"table2","pipeline":[{$match:{"url":{"$eq":"www.example3.com"}}}]}}])