$setDifference (set expression)

APPLIES TO: MongoDB vCore

The $setDifference operator returns a set with elements that exist in the first set but not in the second set. It treats arrays as sets, ignoring duplicate values and element order.

Syntax

The syntax for the $setDifference operator is as follows:

{
  $setDifference: [ <array1>, <array2> ]
}

Parameters

Description
array1 The first array to compare. Elements unique to this array are returned.
array2 The second array to compare against. Elements that exist in both arrays are excluded from the result.

Example

Let's understand the usage with sample JSON from the stores dataset.

{
  "_id": "40d6f4d7-50cd-4929-9a07-0a7a133c2e74",
  "name": "Proseware, Inc. | Home Entertainment Hub - East Linwoodbury",
  "sales": {
    "salesByCategory": [
      {
        "categoryName": "Sound Bars",
        "totalSales": 2120
      },
      {
        "categoryName": "Home Theater Projectors",
        "totalSales": 45004
      },
      {
        "categoryName": "Game Controllers",
        "totalSales": 43522
      },
      {
        "categoryName": "Remote Controls",
        "totalSales": 28946
      },
      {
        "categoryName": "VR Games",
        "totalSales": 32272
      }
    ]
  },
  "promotionEvents": [
    {
      "eventName": "Massive Markdown Mania",
      "discounts": [
        {
          "categoryName": "DVD Players",
          "discountPercentage": 14
        },
        {
          "categoryName": "Media Players",
          "discountPercentage": 21
        },
        {
          "categoryName": "Televisions",
          "discountPercentage": 22
        }
      ]
    }
  ]
}

Example 1: Find categories sold but not discounted

The example checks for product categories with sales data but aren't providing any discounts.

db.stores.aggregate([
  { $match: {"_id": "40d6f4d7-50cd-4929-9a07-0a7a133c2e74"} },
  {
    $project: {
      name: 1,
      soldCategories: "$sales.salesByCategory.categoryName",
      discountedCategories: {
        $reduce: {
          input: "$promotionEvents",
          initialValue: [],
          in: {
            $concatArrays: ["$$value", "$$this.discounts.categoryName"]
          }
        }
      }
    }
  },
  {
    $project: {
      name: 1,
      soldCategories: 1,
      discountedCategories: 1,
      categoriesWithoutDiscounts: {
        $setDifference: ["$soldCategories", "$discountedCategories"]
      }
    }
  }
])

The query output shows categories which are sold but never discounted.

{
  "_id": "40d6f4d7-50cd-4929-9a07-0a7a133c2e74",
  "name": "Proseware, Inc. | Home Entertainment Hub - East Linwoodbury",
  "soldCategories": [
    "Sound Bars",
    "Home Theater Projectors",
    "Game Controllers",
    "Remote Controls",
    "VR Games"
  ],
  "discountedCategories": [
    "DVD Players",
    "Projector Lamps",
    "Media Players",
    "Blu-ray Players",
    "Home Theater Systems",
    "Televisions"
  ],
  "categoriesWithoutDiscounts": [
    "Sound Bars",
    "Home Theater Projectors",
    "Game Controllers",
    "Remote Controls",
    "VR Games"
  ]
}

Example 2: Compare staff distribution types

Find the difference between two hypothetical staff requirement lists.

db.stores.aggregate([
  { $match: {"_id": "40d6f4d7-50cd-4929-9a07-0a7a133c2e74"} },
  {
    $project: {
      name: 1,
      requiredSkills: ["Sales", "Customer Service", "Technical Support", "Inventory Management"],
      availableSkills: ["Sales", "Customer Service", "Marketing", "Administration"],
      missingSkills: {
        $setDifference: [
          ["Sales", "Customer Service", "Technical Support", "Inventory Management"],
          ["Sales", "Customer Service", "Marketing", "Administration"]
        ]
      }
    }
  }
])

The query returns the skills that are required but not available.

{
  "_id": "40d6f4d7-50cd-4929-9a07-0a7a133c2e74",
  "name": "Proseware, Inc. | Home Entertainment Hub - East Linwoodbury",
  "requiredSkills": [
    "Sales",
    "Customer Service", 
    "Technical Support",
    "Inventory Management"
  ],
  "availableSkills": [
    "Sales",
    "Customer Service",
    "Marketing",
    "Administration"
  ],
  "missingSkills": [
    "Technical Support",
    "Inventory Management"
  ]
}

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