通过

$setDifference （set 表达式）

适用对象： MongoDB vCore

该 $setDifference 运算符返回一个集，其中包含存在于第一个集中但不存在于第二个集中的元素。 它将数组视为集，忽略重复值和元素顺序。

语法

$setDifference 运算符的语法如下所示：

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

参数

DESCRIPTION
array1 要比较的第一个数组。 返回此数组特有的元素。
array2 要与之进行比较的第二个数组。 这两个数组中存在的元素都从结果中排除。

示例：

让我们了解数据集中示例 JSON 的 stores 用法。

{
  "_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
        }
      ]
    }
  ]
}

示例 1：查找已售出但未折扣的类别

该示例使用销售数据检查产品类别，但不提供任何折扣。

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"]
      }
    }
  }
])

查询输出显示已销售但从未打折扣的类别。

{
  "_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"
  ]
}

示例 2：比较员工分配类型

查找两个假设员工要求列表之间的差异。

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"]
        ]
      }
    }
  }
])

该查询返回必需但不可用的技能。

{
  "_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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