Aggregations and filters in MongoDB

Aggregations and filters in MongoDB

1. Filters in MongoDB

Filters are used in queries to select documents that match certain conditions. You generally use find() for this.

Example:

Suppose we have a collection employees:

[
  { "_id": 1, "name": "Alice", "age": 30, "department": "HR", "salary": 5000 },
  { "_id": 2, "name": "Bob", "age": 25, "department": "IT", "salary": 6000 },
  { "_id": 3, "name": "Charlie", "age": 28, "department": "IT", "salary": 7000 },
  { "_id": 4, "name": "David", "age": 35, "department": "Finance", "salary": 8000 }
]

Filter Example:

  • Find all employees in the IT department:
db.employees.find({ department: "IT" })

Result:

[
  { "_id": 2, "name": "Bob", "age": 25, "department": "IT", "salary": 6000 },
  { "_id": 3, "name": "Charlie", "age": 28, "department": "IT", "salary": 7000 }
]
  • Find employees with salary greater than 6000:
db.employees.find({ salary: { $gt: 6000 } })

Result:

[
  { "_id": 3, "name": "Charlie", "age": 28, "department": "IT", "salary": 7000 },
  { "_id": 4, "name": "David", "age": 35, "department": "Finance", "salary": 8000 }
]

2. Aggregations in MongoDB

Aggregation allows you to process data records and return computed results. MongoDB provides the aggregate() function with a pipeline of stages.

Common Stages:

  • $match: Filter documents (like find)
  • $group: Group documents by a field and apply aggregate functions
  • $sort: Sort the results
  • $project: Include/exclude or compute new fields
  • $limit / $skip: Limit or skip results

Aggregation Example 1: Total salary by department

db.employees.aggregate([
  { $group: { _id: "$department", totalSalary: { $sum: "$salary" } } }
])

Result:

[
  { "_id": "HR", "totalSalary": 5000 },
  { "_id": "IT", "totalSalary": 13000 },
  { "_id": "Finance", "totalSalary": 8000 }
]

Explanation:

  • $group groups documents by department.
  • $sum adds the salary for each group.

Aggregation Example 2: Filter + Aggregate

Find total salary for IT employees only:

db.employees.aggregate([
  { $match: { department: "IT" } },   // Filter
  { $group: { _id: "$department", totalSalary: { $sum: "$salary" } } }
])

Result:

[
  { "_id": "IT", "totalSalary": 13000 }
]

Explanation:

  • $match filters documents first.
  • $group calculates the total salary for IT employees.

Aggregation Example 3: Sorting and projecting

Find average salary per department and sort by highest average:

db.employees.aggregate([
  { $group: { _id: "$department", avgSalary: { $avg: "$salary" } } },
  { $sort: { avgSalary: -1 } }
])

Result:

[
  { "_id": "Finance", "avgSalary": 8000 },
  { "_id": "IT", "avgSalary": 6500 },
  { "_id": "HR", "avgSalary": 5000 }
]

✅ Key takeaway:

  • Filters (find) are for simple queries.
  • Aggregations (aggregate) are for advanced computations like sums, averages, grouping, sorting, and filtering together.

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