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$cond (conditional expression)

The $cond operator is used to evaluate a condition and return one of two expressions based on the result. It's similar to the ternary operator in many programming languages. The $cond operator can be used within aggregation pipelines to add conditional logic to your queries.

Syntax

The basic syntax for the $cond operator is:

{
   $cond: {
      if: <boolean-expression>,
      then: <true-case>,
      else: <false-case>
   }
}

Parameters

Description
if A boolean expression that is evaluated.
then The expression to return if the if condition evaluates to true.
else The expression to return if the if condition evaluates to false.

Examples

Let's understand the usage with the following sample json.

{
  "_id": "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5",
   "name": "Lakeshore Retail | DJ Equipment Stop - Port Cecile",
  "location": {
    "lat": 60.1441,
    "lon": -141.5012
  },
  "staff": {
    "totalStaff": {
      "fullTime": 2,
      "partTime": 0
    }
  },
  "sales": {
    "salesByCategory": [
      {
        "categoryName": "DJ Headphones",
        "totalSales": 35921
      }
    ],
    "fullSales": 3700
  },
  "promotionEvents": [
    {
      "eventName": "Bargain Blitz Days",
      "promotionalDates": {
        "startDate": {
          "Year": 2024,
          "Month": 3,
          "Day": 11
        },
        "endDate": {
          "Year": 2024,
          "Month": 2,
          "Day": 18
        }
      },
      "discounts": [
        {
          "categoryName": "DJ Turntables",
          "discountPercentage": 18
        },
        {
          "categoryName": "DJ Mixers",
          "discountPercentage": 15
        }
      ]
    }
  ],
  "tag": [
    "#ShopLocal",
    "#SeasonalSale",
    "#FreeShipping",
    "#MembershipDeals"
  ]
}

Example 1: Determine high sales categories

To determine if the sales for each category are considered "high" or "low" based on a threshold value of $250,000.

db.stores.aggregate([
  {
    $project: {
      _id: 0,
      storeId: "$storeId",
      category: "$sales.salesByCategory.categoryName",
      sales: "$sales.salesByCategory.totalSales",
      salesCategory: {
        $cond: {
          if: { $gte: ["$sales.salesByCategory.totalSales", 250000] },
          then: "High",
          else: "Low"
        }
      }
    }
  },
  // Limit the result to the first 3 documents
  { $limit: 3 }  
])

This query would return the following document.

[
  {
    "sales": [ 35921, 1000 ],
    "category": [ "DJ Headphones", "DJ Cables" ],
    "salesCategory": "High"
  },
  { "sales": [ 4760 ], "category": [ "Guitars" ], "salesCategory": "High" },
  {
    "sales": [
      14697, 44111,
      37854, 46211,
       7269, 25451,
      21083
    ],
    "category": [
      "Washcloths",
      "Innerspring Mattresses",
      "Microfiber Towels",
      "Shower Curtains",
      "Bathrobes",
      "Tablecloths",
      "Bath Accessories"
    ],
    "salesCategory": "High"
  }
]

Example 2: Determine full-time or part-time dominance

To determine whether a store employs more full-time or part-time staff.

db.stores.aggregate([
  {
    $project: {
      name: 1,
      staffType: {
        $cond: {
          if: { $gte: ["$staff.totalStaff.fullTime", "$staff.totalStaff.partTime"] },
          then: "More Full-Time",
          else: "More Part-Time"
        }
      }
    }
  },
  // Limit the result to the first 3 documents
  { $limit: 3 }  
])

This query would return the following document.

[
  {
    "_id": "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5",
    "name": "Lakeshore Retail | DJ Equipment Stop - Port Cecile",
    "staffType": "More Full-Time"
  },
  {
    "_id": "649626c9-eda1-46c0-a27f-dcee19d97f41",
    "name": "VanArsdel, Ltd. | Musical Instrument Outlet - East Cassie",
    "staffType": "More Full-Time"
  },
  {
    "_id": "8345de34-73ec-4a99-9cb6-a81f7b145c34",
    "name": "Northwind Traders | Bed and Bath Place - West Oraland",
    "staffType": "More Part-Time"
  }
]

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