Aggregation Operators in MongoDB

From the MongoDB cheat sheet · Aggregation Pipeline · verified Jul 2026

Aggregation Operators

Operators for calculations and transformations

javascript

// Group accumulator operators
db.sales.aggregate([
  { $group: {
    _id: "$category",
    total: { $sum: "$amount" },
    average: { $avg: "$amount" },
    min: { $min: "$amount" },
    max: { $max: "$amount" },
    count: { $sum: 1 },
    items: { $push: "$item" },
    uniqueItems: { $addToSet: "$item" }
  }}
])

// String operators
{ $project: {
  upper: { $toUpper: "$name" },
  lower: { $toLower: "$name" },
  substring: { $substr: ["$name", 0, 3] },
  concat: { $concat: ["$first", " ", "$last"] }
}}

💡 $sum: 1 counts documents

📌 Use $$ for variables in expressions

⚡ $cond for if-then-else logic

🟢 Essential for data analysis

🔗 Related: $map, $reduce for array processing

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