1) $push operator in MongoDB
$push operator returns an array of all values in the group from collection in MongoDB.
2) Find all salary of all employee with same firstName in array from collection in MongoDB >
Step 1 - We will group employee by firstName and find all salary in each group.
Step 2- We will use aggregate() method, $group operator and $push operator
> db.employee.aggregate([{$group : {_id : "$firstName", salary_all : {$push : "$salary"}}}])
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Output >
{ "_id" : "ank", "salary_all" : [ 1000, 2000 ] }
{ "_id" : "sag", "salary_all" : [ 3000, 4000 ] }
{ "_id" : "neh", "salary_all" : [ 5000, 5000 ] }
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3) Find all salary of all employee in array from collection in MongoDB >
Step 1 - We will group employee by null (nothing) and find all salary in group (there will be only 1 group - i.e. whole documents of collection)
Step 2- We will use aggregate() method, $group operator, $push operator and _id : null
> db.employee.aggregate([{$group : {_id : null, salary_all : {$push : "$salary"}}}])
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Output >
{ "_id" : null, "salary_all" : [ 1000, 2000, 3000, 4000, 5000, 5000 ] }
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4) How to write aggregate query with where clause in MongoDB >
Find all salary of all employee with same firstName where salary > 2000 in array from collection in MongoDB >
Step 1 - We will find document where salary > 2000
Step 2 - We will use $match operator
Step 3 - Then we will group employee by firstName and find all salary in each group.
Step 4 - We will use aggregate() method, $group operator, and _id : null
> db.employee.aggregate([
{ $match: { salary : { $gt: 2000} } },
{$group : {_id : "$firstName", salary_all : {$push : "$salary"}}}])
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Output >
{ "_id" : "sag", "salary_all" : [ 3000, 4000 ] }
{ "_id" : "neh", "salary_all" : [ 5000, 5000 ] }
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