$min (aggregation)

Null or Missing ValuesNull或缺少值

If some, but not all, documents for the $min operation have either a null value for the field or are missing the field, the $min operator only considers the non-null and the non-missing values for the field.如果$min操作的某些（但不是所有）文档的字段值为空或缺少字段，则$min运算符仅考虑字段的非空值和非缺失值。

If all documents for the $min operation have null value for the field or are missing the field, the $min operator returns null for the minimum value.如果$min操作的所有文档的字段值都为null或缺少该字段，则$min运算符将返回null作为最小值。

Array Operand数组操作数

In the $group and $setWindowFields stages, if the expression resolves to an array, $min does not traverse the array and compares the array as a whole.在$group和$setWindowFields阶段中，如果表达式解析为数组，则$min不会遍历数组并将数组作为一个整体进行比较。

In the other supported stages:在其他受支持的阶段中：

• With a single expression as its operand, if the expression resolves to an array, $min traverses into the array to operate on the numerical elements of the array to return a single value.使用单个表达式作为操作数，如果表达式解析为数组，$min将遍历数组，对数组的数字元素进行运算，以返回单个值。
• With a list of expressions as its operand, if any of the expressions resolves to an array, $min does not traverse into the array but instead treats the array as a non-numerical value.使用表达式列表作为操作数，如果任何表达式解析为数组，$min不会遍历数组，而是将数组视为非数值。

Use in $group Stage在$group Stage中使用

Consider a sales collection with the following documents:考虑使用以下文档的sales集合：

{ "_id" : 1, "item" : "abc", "price" : 10, "quantity" : 2, "date" : ISODate("2014-01-01T08:00:00Z") }
{ "_id" : 2, "item" : "jkl", "price" : 20, "quantity" : 1, "date" : ISODate("2014-02-03T09:00:00Z") }
{ "_id" : 3, "item" : "xyz", "price" : 5, "quantity" : 5, "date" : ISODate("2014-02-03T09:05:00Z") }
{ "_id" : 4, "item" : "abc", "price" : 10, "quantity" : 10, "date" : ISODate("2014-02-15T08:00:00Z") }
{ "_id" : 5, "item" : "xyz", "price" : 5, "quantity" : 10, "date" : ISODate("2014-02-15T09:05:00Z") }

Grouping the documents by the item field, the following operation uses the $min accumulator to compute the minimum amount and minimum quantity for each grouping.以下操作按item字段对文档进行分组，使用$min累加器计算每个分组的最小金额和最小数量。

db.sales.aggregate(
   [
     {
       $group:
         {
           _id: "$item",
           minQuantity: { $min: "$quantity" }
         }
     }
   ]
)

The operation returns the following results:该操作返回以下结果：

{ "_id" : "xyz", "minQuantity" : 5 }
{ "_id" : "jkl", "minQuantity" : 1 }
{ "_id" : "abc", "minQuantity" : 2 }

Use in $project Stage在$project阶段中使用

A collection students contains the following documents:集合students包含以下文档：

{ "_id": 1, "quizzes": [ 10, 6, 7 ], "labs": [ 5, 8 ], "final": 80, "midterm": 75 }
{ "_id": 2, "quizzes": [ 9, 10 ], "labs": [ 8, 8 ], "final": 95, "midterm": 80 }
{ "_id": 3, "quizzes": [ 4, 5, 5 ], "labs": [ 6, 5 ], "final": 78, "midterm": 70 }

The following example uses the $min in the $project stage to calculate the minimum quiz scores, the minimum lab scores, and the minimum of the final and the midterm:以下示例使用$project阶段中的$min来计算最低测验分数、最低实验室分数以及期末和期中考试的最低分数：

db.students.aggregate([
   { $project: { quizMin: { $min: "$quizzes"}, labMin: { $min: "$labs" }, examMin: { $min: [ "$final", "$midterm" ] } } }
])

The operation results in the following documents:该操作产生以下文档：

{ "_id" : 1, "quizMin" : 6, "labMin" : 5, "examMin" : 75 }
{ "_id" : 2, "quizMin" : 9, "labMin" : 8, "examMin" : 80 }
{ "_id" : 3, "quizMin" : 4, "labMin" : 5, "examMin" : 70 }

Use in $setWindowFields Stage在$setWindowFields阶段中使用

Create a cakeSales collection that contains cake sales in the states of California (CA) and Washington (WA):创建包含加利福尼亚州（CA）和华盛顿州（WA）蛋糕销售的cakeSales集合：

db.cakeSales.insertMany( [
   { _id: 0, type: "chocolate", orderDate: new Date("2020-05-18T14:10:30Z"),
     state: "CA", price: 13, quantity: 120 },
   { _id: 1, type: "chocolate", orderDate: new Date("2021-03-20T11:30:05Z"),
     state: "WA", price: 14, quantity: 140 },
   { _id: 2, type: "vanilla", orderDate: new Date("2021-01-11T06:31:15Z"),
     state: "CA", price: 12, quantity: 145 },
   { _id: 3, type: "vanilla", orderDate: new Date("2020-02-08T13:13:23Z"),
     state: "WA", price: 13, quantity: 104 },
   { _id: 4, type: "strawberry", orderDate: new Date("2019-05-18T16:09:01Z"),
     state: "CA", price: 41, quantity: 162 },
   { _id: 5, type: "strawberry", orderDate: new Date("2019-01-08T06:12:03Z"),
     state: "WA", price: 43, quantity: 134 }
] )

This example uses $min in the $setWindowFields stage to output the minimum quantity of cake sales for each state:此示例使用$setWindowFields阶段中的$min来输出每个状态的最小蛋糕销售数量：

db.cakeSales.aggregate( [
   {
      $setWindowFields: {
         partitionBy: "$state",
         sortBy: { orderDate: 1 },
         output: {
            minimumQuantityForState: {
               $min: "$quantity",
               window: {
                  documents: [ "unbounded", "current" ]
               }
            }
         }
      }
   }
] )

In the example:在示例中：

• partitionBy: "$state" partitions the documents in the collection by state. 按state划分集合中的文档。There are partitions for CA and WA.CA和WA有分区。
• sortBy: { orderDate: 1 } sorts the documents in each partition by orderDate in ascending order (1), so the earliest orderDate is first.按orderDate按升序（1）对每个分区中的文档进行排序，因此最早的orderDate是第一个。

• output sets the minimumQuantityForState field to the minimum quantity value using $min that is run in a documents window.output使用在文档窗口中运行的$min将minimumQuantityForState字段设置为最小数量值。

  The window contains documents between an unbounded lower limit and the current document in the output. 该窗口包含处于unbounded下限和输出中current文档之间的文档。This means $min returns the minimum quantity for the documents between the beginning of the partition and the current document.这意味着$min返回分区开始和当前文档之间文档的最小quantity。

In this output, the minimum quantity for CA and WA is shown in the minimumQuantityForState field:在此输出中，CA和WA的最小quantity显示在minimumQuantityForState字段中：

{ "_id" : 4, "type" : "strawberry", "orderDate" : ISODate("2019-05-18T16:09:01Z"),
  "state" : "CA", "price" : 41, "quantity" : 162, "minimumQuantityForState" : 162 }
{ "_id" : 0, "type" : "chocolate", "orderDate" : ISODate("2020-05-18T14:10:30Z"),
  "state" : "CA", "price" : 13, "quantity" : 120, "minimumQuantityForState" : 120 }
{ "_id" : 2, "type" : "vanilla", "orderDate" : ISODate("2021-01-11T06:31:15Z"),
  "state" : "CA", "price" : 12, "quantity" : 145, "minimumQuantityForState" : 120 }
{ "_id" : 5, "type" : "strawberry", "orderDate" : ISODate("2019-01-08T06:12:03Z"),
  "state" : "WA", "price" : 43, "quantity" : 134, "minimumQuantityForState" : 134 }
{ "_id" : 3, "type" : "vanilla", "orderDate" : ISODate("2020-02-08T13:13:23Z"),
  "state" : "WA", "price" : 13, "quantity" : 104, "minimumQuantityForState" : 104 }
{ "_id" : 1, "type" : "chocolate", "orderDate" : ISODate("2021-03-20T11:30:05Z"),
  "state" : "WA", "price" : 14, "quantity" : 140, "minimumQuantityForState" : 104 }