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- $mul
$mul¶
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Definition¶
- $mul¶
2.6 新版功能.
Multiply the value of a field by a number. To specify a $mul expression, use the following prototype:
{ $mul: { field: <number> } }
The field to update must contain a numeric value.
To specify a <field> in an embedded document or in an array, use dot notation.
Behavior¶
Missing Field¶
If the field does not exist in a document, $mul creates the field and sets the value to zero of the same numeric type as the multiplier.
Atomic¶
$mul is an atomic operation within a single document.
Mixed Type¶
Multiplication with values of mixed numeric types (32-bit integer, 64-bit integer, float) may result in conversion of numeric type. For multiplication with values of mixed numeric types, the following type conversion rules apply:
| 32-bit Integer | 64-bit Integer | Float | |
|---|---|---|---|
| 32-bit Integer | 32-bit or 64-bit Integer | 64-bit Integer | Float |
| 64-bit Integer | 64-bit Integer | 64-bit Integer | Float |
| Float | Float | Float | Float |
注解
- If the product of two 32-bit integers exceeds the maximum value for a 32-bit integer, the result is a 64-bit integer.
- Integer operations of any type that exceed the maximum value for a 64-bit integer produce an error.
Examples¶
Multiply the Value of a Field¶
Consider a collection products with the following document:
{ _id: 1, item: "ABC", price: 10.99 }
The following db.collection.update() operation updates the document, using the $mul operator to multiply the value in the price field by 1.25:
db.products.update(
{ _id: 1 },
{ $mul: { price: 1.25 } }
)
The operation results in the following document, where the new value of the price field 13.7375 reflects the original value 10.99 multiplied by 1.25:
{ _id: 1, item: "ABC", price: 13.7375 }
Apply $mul Operator to a Non-existing Field¶
Consider a collection products with the following document:
{ _id: 2, item: "Unknown" }
The following db.collection.update() operation updates the document, applying the $mul operator to the field price that does not exist in the document:
db.products.update(
{ _id: 2 },
{ $mul: { price: NumberLong(100) } }
)
The operation results in the following document with a price field set to value 0 of numeric type NumberLong, the same type as the multiplier:
{ "_id" : 2, "item" : "Unknown", "price" : NumberLong(0) }
Multiply Mixed Numeric Types¶
Consider a collection products with the following document:
{ _id: 3, item: "XYZ", price: NumberLong(10) }
The following db.collection.update() operation uses the $mul operator to multiply the value in the price field NumberLong(10) by NumberInt(5):
db.products.update(
{ _id: 3 },
{ $mul: { price: NumberInt(5) } }
)
The operation results in the following document:
{ "_id" : 3, "item" : "XYZ", "price" : NumberLong(50) }
The value in the price field is of type NumberLong. See Multiplication Type Conversion Rules for details.