A.db.test.remove({hello:"world"},1)B.db.test.remove({hello:"world"},true)C.db.test.deleteOne({hello:"world"})D.db.test.delete({hello:"world"})
A.gorup_byB.by_groupC.user_byD.by_user
A.show detablesB.show dbsC.show db.sD.show collections