Database

db={"employee":[{"FNAME":"John","LNAME":"Smith","SSN":"123456789","SALARY":30000,"SUPERSSN":"333445555"},{"FNAME":"Franklin","LNAME":"Wong","SSN":"333445555","SALARY":40000,"SUPERSSN":"888665555"},{"FNAME":"Joyce","LNAME":"English","SSN":"453453453","SALARY":25000,"SUPERSSN":"333445555"},{"FNAME":"Ramesh","LNAME":"Narayan","SSN":"666884444","SALARY":38000,"SUPERSSN":"333445555"},{"FNAME":"James","LNAME":"Borg","SSN":"888665555","SALARY":55000,"SUPERSSN":"","DNO":1},{"FNAME":"Jennifer","LNAME":"Wallace","SSN":"987654321","SALARY":43000,"SUPERSSN":"888665555"},{"FNAME":"Ahmad","LNAME":"Jabbar","SSN":"987987987","SALARY":25000,"SUPERSSN":"987654321"},{"FNAME":"Alicia","LNAME":"Zelaya","SSN":"999887777","SALARY":25000,"SUPERSSN":"987654321"},{"FNAME":"John","LNAME":"Smith","SSN":"123456789","SALARY":30000,"SUPERSSN":"333445555"},{"FNAME":"Franklin","LNAME":"Wong","SSN":"333445555","SALARY":40000,"SUPERSSN":"888665555"},{"FNAME":"Joyce","LNAME":"English","SSN":"453453453","SALARY":25000,"SUPERSSN":"333445555"},{"FNAME":"Ramesh","LNAME":"Narayan","SSN":"666884444","SALARY":38000,"SUPERSSN":"333445555"},{"FNAME":"James","LNAME":"Borg","SSN":"888665555","SALARY":55000,"SUPERSSN":"","DNO":1},{"FNAME":"Jennifer","LNAME":"Wallace","SSN":"987654321","SALARY":43000,"SUPERSSN":"888665555"},{"FNAME":"Ahmad","LNAME":"Jabbar","SSN":"987987987","SALARY":25000,"SUPERSSN":"987654321"},{"FNAME":"Alicia","LNAME":"Zelaya","SSN":"999887777","SALARY":25000,"SUPERSSN":"987654321"}],"works_on":[{"ESSN":"123456789","PNO":1,"HOURS":32.5},{"ESSN":"123456789","PNO":2,"HOURS":7.5},{"ESSN":"333445555","PNO":2,"HOURS":10},{"ESSN":"333445555","PNO":3,"HOURS":10},{"ESSN":"333445555","PNO":10,"HOURS":10},{"ESSN":"333445555","PNO":20,"HOURS":10},{"ESSN":"453453453","PNO":1,"HOURS":20},{"ESSN":"453453453","PNO":2,"HOURS":20},{"ESSN":"666884444","PNO":3,"HOURS":40},{"ESSN":"888665555","PNO":20,"HOURS":0},{"ESSN":"987654321","PNO":20,"HOURS":15},{"ESSN":"987654321","PNO":30,"HOURS":20},{"ESSN":"987987987","PNO":10,"HOURS":35.5},{"ESSN":"987987987","PNO":30,"HOURS":5.5},{"ESSN":"999887777","PNO":10,"HOURS":10},{"ESSN":"999887777","PNO":30,"HOURS":30}]}

Query

db.employee.aggregate([{$group:{_id:"$SSN",FNAME:{$first:"$FNAME"},LNAME:{$first:"LNAME"}}},{$lookup:{from:"works_on",localField:"_id",foreignField:"ESSN",as:"HOURS"}},{$project:{_id:0,FNAME:1,LNAME:1,HOURS:{$sum:"$HOURS.HOURS"}}}])

Result