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03 MongoDB Queries

Create

Create a database: use database_name Create a collection: db.createCollection(name, options)

MongoDB provides two methods to create new documents.

  • db.collection.insertOne(document) inserts one document in the selected collection.
  • db.collection.insertMany([document1, ...]) inserts a group of documents in the selected collection.

Read

  • db.collection.findOne(query, projection)
  • db.collection.find(query, projection)
Parameter Type Description
query document Optional. Specifies selection filter using query operators).
projection document Optional. Specifies the fields to return in the documents that match the query filter.

Update

  • db.collection.updateOne(filter, update) update one document in the selected collection.
  • db.collection.updateMany(filter, update) update a group of documents in the selected collection.

Delete

  • db.dropDatabase() delete database.
  • db.collection.drop() delete a collection.
  • db.collection.deleteOne(filter) delete one document in the selected collection.
  • db.collection.deleteMany(filter) delete a group of documents in the selected collection.

Sort & limit

The limit() function will limit the amount of documents retrieved by a query.

The sort() function organizes the retrieved documents depending on the key-value arguments it receives. Documents can be sorted in an ascending (value = 1) or descending (value = -1) order.

Aggregations

Aggregations are operations that are used to aggregate with respect to one or more fields. MongoDB provides SQL-like aggregation functionality.

Aggregations are pipelines, therefore the operations included in the query are performed sequentially.

Rather than the usual db.collection.find() operations, aggregations requires the usage of the db.collection.aggregate(pipeline) function.

Such function includes two different parts, a filter one (i.e., the conditions w.r.t. the documents are collected) and the aggregation one (i.e., the part that manages the aggregate operations).

The aggregation function supported are various, the main ones are

  • $min returns the lowest expression value for each group
  • $max returns the highest expression value for each group
  • $avg returns an average of numerical values. Ignores non-numeric values.
  • $sum returns a sum of numerical values. Ignores non-numeric values.

An example with the following documents:

{ _id: 1, cust_id: "abc1", ord_date: ISODate("2012-11-02T17:04:11.102Z"), status: "A", amount: 50 }
{ _id: 2, cust_id: "xyz1", ord_date: ISODate("2013-10-01T17:04:11.102Z"), status: "A", amount: 100 }
{ _id: 3, cust_id: "xyz1", ord_date: ISODate("2013-10-12T17:04:11.102Z"), status: "D", amount: 25 }
{ _id: 4, cust_id: "xyz1", ord_date: ISODate("2013-10-11T17:04:11.102Z"), status: "D", amount: 125 }
{ _id: 5, cust_id: "abc1", ord_date: ISODate("2013-11-12T17:04:11.102Z"), status: "A", amount: 25 }

Group by and Calculate a Sum:

db.orders.aggregate([
    { $match: { status: "A" } },
    { $group: { _id: "$cust_id", total: { $sum: "$amount" } } },
    { $sort: { total: -1 } }
])
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