$first operator in MongoDB

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$first operator in MongoDB >
$first operator returns the first document from documents in the collection in MongoDB.
1) First let's create insert documents in collection in MongoDB


1.1) First, Let's create new collection and insert document >
db.employee.insert({_id : 1,  firstName:"ank", salary : 1000 })
db.employee.insert({_id : 2,  firstName:"ank", salary : 2000 })
db.employee.insert({_id : 3,  firstName:"sag", salary : 3000 })
db.employee.insert({_id : 4,  firstName:"sag", salary : 4000 })
db.employee.insert({_id : 5,  firstName:"neh", salary : 5000 })
Above will create collection (or table) (if collection already exists it will insert documents in it).


1.2) FIND > Query all documents of collection using find() method>
db.employee.find()
Output>
{ "_id" : 1, "firstName" : "ank", "salary" : 1000 }
{ "_id" : 2, "firstName" : "ank", "salary" : 2000 }
{ "_id" : 3, "firstName" : "sag", "salary" : 3000 }
{ "_id" : 4, "firstName" : "sag", "salary" : 4000 }
{ "_id" : 5, "firstName" : "neh", "salary" : 5000 }

2) Find first salary of employee with same firstName in collection in MongoDB >


Step 2.1- We will use aggregate() method, $group operator and $first operator.
Step 2.2 - group employee by firstName and find first salary of each group.


db.employee.aggregate([{$group : {_id : "$firstName", salary_first : {$first : "$salary"}}}])
Output >
{ "_id" : "ank", "salary_first" : 1000 }
{ "_id" : "sag", "salary_first" : 3000 }
{ "_id" : "neh", "salary_first" : 5000 }


3) Find first salary of all employee in collection in MongoDB >


Step 3.1 - We will group employee by null (nothing) and find first salary of group (there will be only 1 group - i.e. whole documents of collection)
Step 3.2- We will use aggregate() method, $group operator, $first operator.
db.employee.aggregate([{$group : {_id : null, salary_first : {$first : "$salary"}}}])
Output >
{ "_id" : null, "salary_first" : 1000 }

4) Summary -
So in this MongoDB tutorial we learned $first operator in MongoDB. $first operator returns the first document from documents in the collection in MongoDB.


Find first salary of employee with same firstName in collection in MongoDB >
db.employee.aggregate([{$group : {_id : "$firstName", salary_first : {$first : "$salary"}}}])


Find first salary of all employee in collection in MongoDB >
db.employee.aggregate([{$group : {_id : null, salary_first : {$first : "$salary"}}}])



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