Difference between revisions of "AGGREGATE examples"

Revision as of 19:52, 16 July 2015

#ENCODING
import io
import sys
sys.stdout = io.TextIOWrapper(sys.stdout.buffer, encoding='utf-16')
#MONGO
from pymongo import MongoClient
client = MongoClient()
client.progzoo.authenticate('scott','tiger')
db = client['progzoo']
#PRETTY
import pprint
pp = pprint.PrettyPrinter(indent=4)

Introducing the aggregation framework

These examples introduce the aggregation framework and its operators. Again we will be using the collection world

$match Allows us to perform queries in a similar way to find()

Show all the details for France

pp.pprint(list(
    db.world.aggregate([
        {"$match":{"name":"France"}}
    ])
))

pp.pprint(list(db.world.aggregate([{"$match":{"name":"France"}}])))

$project Allows us to select what fields to display.
It can also has the ability to insert new fields and allows you to compare fields against each other without using $where

Show the name and population density of all Asian countries. (population/area)

Note that "density" is a new field, made from the result of dividing two existing fields, and that $divide is an aggregate function.

To avoid diving by 0 we do a $match to remove any countries with negligible area, then pipe these results through to $project

pp.pprint(list(
     db.world.aggregate([
        {"$match":{"area":{"$ne":0},"continent":"Asia"}},
        {"$project":{
            "_id":0,
            "name":1,
            "density": {"$divide": ["$population","$area"]}
        }}
    ])
))

pp.pprint(list(db.world.aggregate([{"$match":{"area":{"$ne":0},"continent":"Asia"}},{"$project":{"_id":0,"name":1,"density":{"$divide":["$population","$area"]}}}])))

$sort Allows us to choose how the results are displayed, where 1 is ascending and -1 is descending.
Note that excluding $match is the same as {"$match":{}}

Show the name of all countries in descending order.

pp.pprint(list(
     db.world.aggregate([
        {"$project":{
            "_id":0,
            "name":1,
        }},
        {"$sort":{
            "name":-1
        }}  
    ])
))

pp.pprint(list(db.world.aggregate([{"$project":{"_id":0,"name":1,}},{"$sort":{"name":-1}}])))

Grouping

Grouping allows us to use accumulator operations sum as $sum
All groups must have an _id, but this can be set to null.

$max and $min can be used to get the largest and smallest values in a group.

Show the country with the largest gdp and the one with the lowest

pp.pprint(list(
     db.world.aggregate([
        {"$group":{
            "_id": $name,
            smallest: {"$min": "$gdp"},
            largest:  {"$max": "$gdp"}
        }},
    ])
))

pp.pprint(list(db.world.aggregate([{"$group":{"_id":$name,smallest:{"$min":"$gdp"},largest:{"$max":"$gdp"}}},])))

@@ Line 77: / Line 77: @@
 <div class=ans>
 pp.pprint(list(db.world.aggregate([{"$project":{"_id":0,"name":1,}},{"$sort":{"name":-1}}])))
+</div>
+</div>
+==Grouping==
+Grouping allows us to use accumulator operations sum as <code>$sum</code><br />
+All groups must have an <code>_id</code>, but this can be set to <code>null</code>.
+<div class='extra_space' style='width:1em; height:6em;'></div>
+<div class=q data-lang="py3">
+<code>$max</code> and <code>$min</code> can be used to get the largest and smallest values in a group.
+<p class=strong>Show the country with the largest gdp and the one with the lowest</p>
+<pre class=def>
+pp.pprint(list(
+     db.world.aggregate([
+        {"$group":{
+            "_id": $name,
+            smallest: {"$min": "$gdp"},
+            largest:  {"$max": "$gdp"}
+        }},
+    ])
+))
+</pre>
+<div class=ans>
+pp.pprint(list(db.world.aggregate([{"$group":{"_id":$name,smallest:{"$min":"$gdp"},largest:{"$max":"$gdp"}}},])))
 </div>
 </div>