MongoDB: How to Group By and Count


You can use the following syntax to group by and count in MongoDB:

db.collection.aggregate([
    {$group : {_id:"$field_name", count:{$sum:1}}}
])

Note that field_name is the field you’d like to group by.

The following examples show how to use this syntax with a collection teams with the following documents:

db.teams.insertOne({team: "Mavs", position: "Guard", points: 31})
db.teams.insertOne({team: "Spurs", position: "Guard", points: 22})
db.teams.insertOne({team: "Rockets", position: "Center", points: 19})
db.teams.insertOne({team: "Warriors", position: "Forward", points: 26})
db.teams.insertOne({team: "Cavs", position: "Guard", points: 33})

Example 1: Group By and Count

We can use the following code to group by the ‘position’ field and count the occurrences of each position.

db.teams.aggregate([
    {$group : {_id:"$position", count:{$sum:1}}}
])

This returns the following results:

{ _id: 'Forward', count: 1 }
{ _id: 'Guard', count: 3 }
{ _id: 'Center', count: 1 }

This tells us:

  • The position ‘Forward’ occurs 1 time.
  • The position ‘Guard’ occurs 3 times.
  • The position ‘Center’ occurs 1 time.

Example 2: Group By and Count (Then Sort)

We can use the following code to count the occurrences of each position and automatically sort the results in ascending order:

db.teams.aggregate([
    {$group : {_id:"$position", count:{$sum:1}}},
    {$sort: {count:1}} 
])

This returns the following results:

{ _id: 'Forward', count: 1 }
{ _id: 'Center', count: 1 }
{ _id: 'Guard', count: 3 }

We can use -1 in the count argument to instead sort the results in descending order:

db.teams.aggregate([
    {$group : {_id:"$position", count:{$sum:1}}},
    {$sort: {count:-1}} 
])

This returns the following results:

{ _id: 'Guard', count: 3 }
{ _id: 'Forward', count: 1 }
{ _id: 'Center', count: 1 }

Note: You can find the complete documentation for $group here.

Additional Resources

The following tutorials explain how to perform other common operations in MongoDB:

MongoDB: How to Count Distinct Values in Field
MongoDB: How to Group By Multiple Fields

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