Similar to FROM, specifies the collection to use

Similar to WHERE, specifies which documents to return

Projection variables in SELECT

How many results to return

db.Students2.count('age')
db.Students2.count({'age':{'$exists':True}})

SELECT COUNT(age) FROM Students2

SELECT COUNT(DISTINCT age) FROM Students2

len(db.Students2.distinct('age'))

db.Students2.aggregate([
    {'$group':{'_id':'$department','meanAge':{'$avg':'$age'}}}
])

SELECT MEAN(age) FROM Students2 GROUP BY department;

SELECT MEAN(age) FROM Students2 
WHERE course0='a' OR course1='a' OR course2='a' 
GROUP BY department;

db.Students2.aggregate([
    {'$match':{'courses':{'$in':['a']}}}
    {'$group':{'_id':'$department','meanAge':{'$avg':'$age'}}}
])

db.Students2.aggregate([
    {
     '$group':{
        '_id':{'department':'$department','college':'$college'},
        'meanAge':{'$avg':'$age'}
        }
    }
])

SELECT MEAN(age) FROM Students2 GROUP BY department, college;

db.Students2.aggregate([
    {'$match':{'$text':{'$search':'Miami Florida'}}},
    {'$sort':{'score':{'$meta':'textScore'}}},
    {'$project':{'hometown':1,'name':1}}
])

Colleges

Students2

db.Colleges.aggregate([
    {
     '$lookup':{
        'from':'Students2',
        'localField':'name',
        'foreignField':'college',
        'as':'students'
      }
    }
])

db.Students2.aggregate([
    {
     '$lookup':{
        'from':'Colleges',
        'localField':'college',
        'foreignField':'name',
        'as':'colleges'
      }
    }
])