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$min

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Definition定义

$min

The $min updates the value of the field to a specified value if the specified value is less than the current value of the field. 如果指定值小于字段的当前值，$min会将字段的值更新为指定值。The $min operator can compare values of different types, using the BSON comparison order.$min运算符可以使用BSON比较顺序比较不同类型的值。

{ $min: { <field1>: <value1>, ... } }

To specify a <field> in an embedded document or in an array, use dot notation.要在嵌入文档或数组中指定<field>，请使用点表示法。

Behavior行为

Starting in MongoDB 5.0, update operators process document fields with string-based names in lexicographic order. 从MongoDB 5.0开始，更新运算符按照字典顺序处理具有基于字符串的名称的文档字段。Fields with numeric names are processed in numeric order. 具有数字名称的字段按数字顺序处理。See Update Operators Behavior for details.有关详细信息，请参阅更新运算符行为。

If the field does not exist, the $min operator sets the field to the specified value.如果该字段不存在，$min运算符会将该字段设置为指定值。

For comparisons between values of different types, such as a number and a null, $min uses the BSON comparison order.对于不同类型的值（如数字和null）之间的比较，$min使用BSON比较顺序。

Starting in MongoDB 5.0, mongod no longer raises an error when you use an update operator like $min with an empty operand expression ( { } ). 从MongoDB 5.0开始，当您将$min之类的更新运算符与空操作数表达式（{ }）一起使用时，mongod不再引发错误。An empty update results in no changes and no oplog entry is created (meaning that the operation is a no-op).空更新不会导致任何更改，也不会创建oplog条目（这意味着该操作是无操作）。

Examples实例

Use $min to Compare Numbers使用$min比较数字

Create the scores collection:创建scores集合：

db.scores.insertOne( { _id: 1, highScore: 800, lowScore: 200 } )

The lowScore for the document currently has the value 200. 文档的lowScore当前的值为200。The following operation uses $min to compare 200 to the specified value 150 and updates the value of lowScore to 150 since 150 is less than 200:以下操作使用$min将200与指定值150进行比较，并将lowScore的值更新为150，因为150小于200：

db.scores.updateOne( { _id: 1 }, { $min: { lowScore: 150 } } )

The scores collection now contains the following modified document:scores集合现在包含以下修改后的文档：

{ _id: 1, highScore: 800, lowScore: 150 }

The next operation has no effect since the current value of the field lowScore, i.e 150, is less than 250:由于字段lowScore（即150）的当前值小于250:

db.scores.updateOne( { _id: 1 }, { $min: { lowScore: 250 } } )

The document remains unchanged in the scores collection:该文档在scores集合中保持不变：

{ _id: 1, highScore: 800, lowScore: 150 }

Use $min to Compare Dates使用$min比较日期

Create the tags collection:创建tags集合：

db.tags.insertOne(
   {
     _id: 1,
     desc: "crafts",
     dateEntered: ISODate("2013-10-01T05:00:00Z"),
     dateExpired: ISODate("2013-10-01T16:38:16Z")
   }
)

The following operation compares the current value of the dateEntered field, i.e. ISODate("2013-10-01T05:00:00Z"), with the specified date new Date("2013-09-25") to determine whether to update the field:以下操作将dateEntered字段的当前值，即ISODate("2013-10-01T05:00:00Z")与指定的日期new Date("2013-09-25")进行比较，以确定是否更新该字段：

db.tags.updateOne(
   { _id: 1 },
   { $min: { dateEntered: new Date("2013-09-25") } }
)

The operation updates the dateEntered field:操作会更新dateEntered字段：

{
  _id: 1,
  desc: "crafts",
  dateEntered: ISODate("2013-09-25T00:00:00Z"),
  dateExpired: ISODate("2013-10-01T16:38:16Z")
}
Tip

See also: 另请参阅：