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Author: GoesToEleven

046_mongodb/05_query/README.md

@@ -39,7 +39,7 @@ db.customers.find({$and: [{name:"Bond"}, {age:{$gt:20}}]})
 ```
 db.customers.find({$or: [{name:"Bond"}, {age:67}]})
 db.customers.find({$or: [{name:"Bond"}, {age:{$lt:20}}]})
-db.customers.find({$or: [{name:"Bond"}, {age:{$gt:20}}]})
+db.customers.find({$or: [{name:"Bond"}, {age:{$gt:32}}]})
 ```
 
 ### and or

046_mongodb/12_aggregate/README.md

@@ -0,0 +1,112 @@
+# aggregate
+
+Aggregations operations process data records and return computed results. Aggregation operations group values from multiple documents together, and can perform a variety of operations on the grouped data to return a single result. MongoDB provides three ways to perform aggregation: the aggregation pipeline, the map-reduce function, and single purpose aggregation methods.
+
+[documenation about aggregation](https://docs.mongodb.com/manual/aggregation/)
+
+### single purpose aggregation
+
+[documenation about single purpose aggregation](https://docs.mongodb.com/manual/aggregation/#single-purpose-agg-operations)
+
+There are two functions you can use:
+1. [db.collection.count()](https://docs.mongodb.com/manual/reference/method/db.collection.count/#db.collection.count)
+1. db.collection.distinct()
+
+### examples - count()
+```
+db.oscars.count()
+```
+
+The above is equivalent to
+
+```
+db.oscars.find().count()
+```
+
+```
+db.customers.find({role:"citizen"}).count()
+```
+
+```
+db.customers.find({$or: [{name:"Bond"}, {age:{$gt:32}}]}).count()
+```
+
+### examples - distinct() - setup
+```
+db.inventory.insert([
+{ "_id": 1, "dept": "A", "item": { "sku": "111", "color": "red" }, "sizes": [ "S", "M" ] },
+{ "_id": 2, "dept": "A", "item": { "sku": "111", "color": "blue" }, "sizes": [ "M", "L" ] },
+{ "_id": 3, "dept": "B", "item": { "sku": "222", "color": "blue" }, "sizes": "S" },
+{ "_id": 4, "dept": "A", "item": { "sku": "333", "color": "black" }, "sizes": [ "S" ] }
+])
+```
+
+### examples - distinct()
+
+syntax
+```
+db.collection.distinct(field, query, options)
+```
+
+
+| Parameter | Description |
+| --- | --- | 
+| field | The field for which to return distinct values.
+| query | A query that specifies the documents from which to retrieve the distinct values.
+| options | Optional. A document that specifies the options. See Options.
+
+```
+db.inventory.distinct( "dept" )
+```
+
+embedded field:
+
+```
+db.inventory.distinct( "item.sku" )
+```
+
+```
+db.inventory.distinct( "sizes" )
+```
+
+
+You use different **expressions** (listed below) in aggregate group operations.
+
+![aggregate pipeline](aggregate.png)
+
+```
+db.<collection name>.aggregate([{<match, sort, geoNear>},{<group>}])
+```
+
+[documenation](https://docs.mongodb.com/manual/aggregation/)
+[match, sort, geonear](https://docs.mongodb.com/manual/core/aggregation-pipeline/#aggregation-pipeline-operators-and-performance)
+
+### examples - setup
+```
+db.orders.insert([
+{"cust_id":"A123","amount":500,"status":"A"},
+{"cust_id":"A123","amount":250,"status":"A"},
+{"cust_id":"B212","amount":200,"status":"A"},
+{"cust_id":"A123","amount":300,"status":"D"}
+])
+```
+
+### examples
+```
+db.orders.aggregate([
+{$match:{status:"A"}},
+{$group:{_id: "$cust_id",total: {$sum:"$amount"}}}
+])
+```
+
+
+### expressions
+
+| expression | description |
+
+```
+db.oscars.createIndex({title:1})
+db.oscars.createIndex({releaseYear:1, releaseDay:1})
+```
+
+[learn to create a unique index and more](https://docs.mongodb.com/manual/reference/method/db.collection.createIndex/#db.collection.createIndex)