$max¶

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Definition
Behavior
Examples

Definition¶

$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. The $max operator can compare values of different types, using the BSON comparison order.

The $max operator expression has the form:

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

To specify a <field> in an embedded document or in an array, use dot notation.

Behavior¶

If the field does not exists, the $max operator sets the field to the specified value.

Examples¶

Use `$max` to Compare Numbers¶

Consider the following document in the collection scores:

{ _id: 1, highScore: 800, lowScore: 200 }

The highScore for the document currently has the value 800. The following operation uses $max to compare the 800 and the specified value 950 and updates the value of highScore to 950 since 950 is greater than 800:

db.scores.update( { _id: 1 }, { $max: { highScore: 950 } } )

The scores collection now contains the following modified document:

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

The next operation has no effect since the current value of the field highScore, i.e. 950, is greater than 870:

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

The document remains unchanged in the scores collection:

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

Use `$max` to Compare Dates¶

Consider the following document in the collection tags:

{
  _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, i.e. ISODate("2013-10-01T16:38:16.163Z"), with the specified date new Date("2013-09-30") to determine whether to update the field:

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

The operation does not update the dateExpired field:

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

参见

db.collection.update(), db.collection.findAndModify()

← $min $currentDate →