IF we want to group document by specific expression and want to output for each distinct grouping of the document here we have to
use $group. IF you are familiar with Relational Database like SQL Server, It's work the same as the GROUP BY clause.
Output document contains the _id field which contains the
distinct group by key also output document contains computed fields which grasp
the value of some accumulator expression grouped by the _id.
Syntax:
{
$group: { _id: <expression>, <field1>: { <accumulator1> :
<expression1> }, ... }
|
Accumulator
Operator:
$num
: It's
returns the sum of the numeric value. It can be used in $project also in MongoDB 3.2
version.
$avg
: It returns the average
of numeric values. It can be used in $project also in MongoDB 3.2 version.
$first
: It's
returns a value from the first document for each group.
$last
: It's
returns a value from the first document for each group.
$max
: It's
returns the highest expression value for each group. It can be used in $project also in MongoDB 3.2 version.
$min
: It's
returns the lowest expression value for each group. It can be used in $project also in MongoDB 3.2
version.
$push
: It's
returns an array of expression values for each group.
$addToSet
: It's
returns an array of unique expression values for each group. The order of the array
elements is undefined.
$stdDevPop
: It's
returns the population standard deviation of the input values.
$stdDevSamp
: It's
returns the sample standard deviation of the input values.
Example
:
{
"_id" : 1, "Book Title": "book1",
"price" : 20, "quantity" : 1, "date" :
ISODate("2016-08-05T07:00:00Z") }
{
"_id" : 2, " Book Title " : " book2",
"price" : 10, "quantity" : 2, "date" :
ISODate("2016-08-05T08:00:00Z") }
{
"_id" : 3, " Book Title " : " book3",
"price" : 30, "quantity" : 4, "date" :
ISODate("2016-08-17T10:00:00Z") }
{
"_id" : 4, " Book Title " : " book4",
"price" : 10, "quantity" : 2, "date" :
ISODate("2014-09-01T11:20:39.736Z") }
{
"_id" : 5, " Book Title " : " book5",
"price" : 20, "quantity" : 6, "date" :
ISODate("2014-09-04T20:23:13.331Z") }
|
Group
by Month, and Year
db.BookOrder.aggregate(
[
{
$group : {
_id : { month: { $month:
"$date" , year: { $year: "$date" } },
totalAmount: { $sum: { $multiply:
[ "$price", "$quantity" ] } },
totalQty: { $sum:
"$quantity" },
count: { $sum: 1 }
}
}
]
)
|
Result:
{
"_id" : { "month" : 8, "year" : 2016 }, " totalAmount
" : 60, " totalQty " : 7, "count" : 3 }
{
"_id" : { "month" : 9, "year" : 2016 }, " totalAmount
" : 30, " totalQty " : 8, "count" : 2 }
|
Note
:
_id filed is mandatory, except _id all fields are optional and computed using
accumulator.
