$max

The $max operator returns the highest value from a list of expressions or grouped values.

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📌 Syntax

{ "$max": <expression> }

Used in: - $group (accumulator) - Array contexts - Numeric comparisons

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✅ Base Example – Maximum in Array

📥 Input Document

{ "scores": [85, 92, 78] }

📌 Expression

{ "$max": "$scores" }

📤 Output

92

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🧱 Ecommerce Example – Maximum Product Price Per Brand

📌 Pipeline

[
  { "$unwind": "$items" },
  {
    "$group": {
      "_id": "$items.brand",
      "maxPrice": { "$max": "$items.price" }
    }
  }
]

📥 Input Documents

[
  {
    "items": [
      { "brand": "A", "price": 100 },
      { "brand": "A", "price": 130 },
      { "brand": "B", "price": 90 }
    ]
  }
]

📤 Output

[
  { "_id": "A", "maxPrice": 130 },
  { "_id": "B", "maxPrice": 90 }
]

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🔧 Common Use Cases

• Determine highest price, score, value
• Identify latest timestamps
• Combine with $group for summary statistics

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🔗 Related Operators

• $min, $avg, $sum, $reduce, $first, $last

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🧠 Notes

• Can be used directly in array or accumulator contexts
• Behavior on null values depends on placement/order

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