$max

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Definition定义

$max

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

The $max operator expression has the form:$max运算符表达式的形式如下：

{ $max: { <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，更新运算符以词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 exists, the $max operator sets the field to the specified value.如果该字段不存在，$max运算符将该字段设置为指定值。

Starting in MongoDB 5.0, mongod no longer raises an error when you use an update operator like $max with an empty operand expression ( { } ). 从MongoDB 5.0开始，当使用带有空操作数表达式（{ }）的更新运算符（如$max）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示例

Use $max to Compare Numbers使用$max比较数字

Create the scores collection:创建scores集合：

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

The highScore for the document currently has the value 800. 文档的highScore当前值为800。The following operation:以下操作：

  • Compares the highscore, 800, to the specified value, 950将highScore800与指定值950进行比较
  • Updates highScore to 950 since 950 is greater than 800将highScore更新为950，因为950大于800
db.scores.updateOne( { _id: 1 }, { $max: { highScore: 950 } } )

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

{ _id: 1, highScore: 950, lowScore: 200 }

The next operation has no effect since the value of highScore, 950, is greater than 870:下一个操作没有效果，因为highScore 950的值大于870：

db.scores.updateOne( { _id: 1 }, { $max: { highScore: 870 } } )

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

{ _id: 1, highScore: 950, lowScore: 200 }

Use $max to Compare Dates使用$max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("2013-10-01T16:38:16.163Z")
   }
)

The following operation compares the current value of the dateExpired field, ISODate("2013-10-01T16:38:16.163Z"), with the specified date new Date("2013-09-30") to determine whether to update the field:以下操作将dateExpired字段ISODate("2013-10-01T16:38:16.163Z")的当前值与指定日期new Date("2013-09-30")进行比较，以确定是否更新该字段：

db.tags.updateOne(
   { _id: 1 },
   { $max: { dateExpired: new Date("2013-09-30") } }
)

new Date("2013-09-30") is not the newest date, so the operation does not update the dateExpired field:不是最新日期，因此该操作不会更新dateExpired字段：

{
   _id: 1,
   desc: "decorative arts",
   dateEntered: ISODate("2013-10-01T05:00:00Z"),
   dateExpired: ISODate("2013-10-01T16:38:16.163Z")
}
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