$shift (aggregation)

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定义
行为
示例

定义

版本 5.0 中的新增功能。

$shift

Returns the value from an 表达式(expression) applied to a document in a specified position relative to the current document in the $setWindowFields stage partition.

The $setWindowFields stage sortBy field value determines the document order. For more information on how MongoDB compares fields with different types, see BSON comparison order.

$shift 只能在 $setWindowFields 阶段使用。

$shift 语法：

{
   $shift: {
      output: <output expression>,
      by: <integer>,
      default: <default expression>
   }
}

$shift接受包含以下字段的文档：

字段	说明
输出	Specifies an 表达式(expression) to evaluate and return in the output.
by	Specifies an `integer` with a numeric document position relative to the current document in the output. 例如： `1` specifies the document position after the current document. `-1` specifies the document position before the current document. `-2` specifies the document position that is two positions before the current document.
访问	Specifies an optional default 表达式(expression) to evaluate if the document position is outside of the implicit `$setWindowFields` stage 窗口. The implicit window contains all the documents in the partition. The 访问 expression must evaluate to a constant value. If you do not specify a 访问 expression, `$shift` returns `null` for documents whose positions are outside of the implicit `$setWindowFields` stage window.

行为

$shift returns an error if you specify a 窗口 in the $setWindowFields stage.

示例

创建cakeSales集合，其中包含加利福尼亚州 ( CA ) 和华盛顿州 ( WA ) 的蛋糕销售情况：

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 }
] )

以下示例中使用了 cakeSales 集合。

Shift Using a Positive Integer

This example uses $shift in the $setWindowFields stage to output the quantity of the cake sales from each document following the current document for each state:

db.cakeSales.aggregate( [
   {
      $setWindowFields: {
         partitionBy: "$state",
         sortBy: { quantity: -1 },
         output: {
            shiftQuantityForState: {
               $shift: {
                  output: "$quantity",
                  by: 1,
                  default: "Not available"
               }
            }
         }
      }
   }
] )

在示例中：

partitionBy: "$state" 按 state 对集合中的文档分区。CA 和 WA 都有分区。
sortBy: { quantity: -1 } 按 quantity 以降序 (-1) 对每个分区中的文档进行排序，因此最高的 quantity 位于最前面。

output after sortBy:
- Sets the shiftQuantityForState field to the quantity value from the documents in each state.
- Uses $shift to return the quantity value from the document that follows the current document in the output.
  - The document position is specified using the $shift by integer set to 1.
  - For documents outside of the implicit 窗口, $shift returns "Not available", which is specified using the 访问 expression.

In this example output, the shifted quantity value is shown in the shiftQuantityForState field for each returned document:

{ "_id" : 4, "type" : "strawberry", "orderDate" : ISODate("2019-05-18T16:09:01Z"),
  "state" : "CA", "price" : 41, "quantity" : 162, "shiftQuantityForState" : 145 }
{ "_id" : 2, "type" : "vanilla", "orderDate" : ISODate("2021-01-11T06:31:15Z"),
  "state" : "CA", "price" : 12, "quantity" : 145, "shiftQuantityForState" : 120 }
{ "_id" : 0, "type" : "chocolate", "orderDate" : ISODate("2020-05-18T14:10:30Z"),
  "state" : "CA", "price" : 13, "quantity" : 120, "shiftQuantityForState" : "Not available" }
{ "_id" : 1, "type" : "chocolate", "orderDate" : ISODate("2021-03-20T11:30:05Z"),
  "state" : "WA", "price" : 14, "quantity" : 140, "shiftQuantityForState" : 134 }
{ "_id" : 5, "type" : "strawberry", "orderDate" : ISODate("2019-01-08T06:12:03Z"),
  "state" : "WA", "price" : 43, "quantity" : 134, "shiftQuantityForState" : 104 }
{ "_id" : 3, "type" : "vanilla", "orderDate" : ISODate("2020-02-08T13:13:23Z"),
  "state" : "WA", "price" : 13, "quantity" : 104, "shiftQuantityForState" : "Not available" }

Shift Using a Negative Integer

This example uses $shift in the $setWindowFields stage to output the quantity of the cake sales from each document before the current document for each state:

db.cakeSales.aggregate( [
   {
      $setWindowFields: {
         partitionBy: "$state",
         sortBy: { quantity: -1 },
         output: {
            shiftQuantityForState: {
               $shift: {
                  output: "$quantity",
                  by: -1,
                  default: "Not available"
               }
            }
         }
      }
   }
] )

在示例中：

partitionBy: "$state" 按 state 对集合中的文档分区。CA 和 WA 都有分区。
sortBy: { quantity: -1 } 按 quantity 以降序 (-1) 对每个分区中的文档进行排序，因此最高的 quantity 位于最前面。

output after sortBy:
- Sets the shiftQuantityForState field to the quantity value from the documents in each state.
- Uses $shift to return the quantity value from the document before the current document in the output.
  - The document position is specified using the $shift by integer set to -1.
  - For documents outside of the implicit 窗口, $shift returns "Not available", which is specified using the 访问 expression.

In this example output, the shifted quantity value is shown in the shiftQuantityForState field for each returned document:

{ "_id" : 4, "type" : "strawberry", "orderDate" : ISODate("2019-05-18T16:09:01Z"),
  "state" : "CA", "price" : 41, "quantity" : 162, "shiftQuantityForState" : "Not available" }
{ "_id" : 2, "type" : "vanilla", "orderDate" : ISODate("2021-01-11T06:31:15Z"),
  "state" : "CA", "price" : 12, "quantity" : 145, "shiftQuantityForState" : 162 }
{ "_id" : 0, "type" : "chocolate", "orderDate" : ISODate("2020-05-18T14:10:30Z"),
  "state" : "CA", "price" : 13, "quantity" : 120, "shiftQuantityForState" : 145 }
{ "_id" : 1, "type" : "chocolate", "orderDate" : ISODate("2021-03-20T11:30:05Z"),
  "state" : "WA", "price" : 14, "quantity" : 140, "shiftQuantityForState" : "Not available" }
{ "_id" : 5, "type" : "strawberry", "orderDate" : ISODate("2019-01-08T06:12:03Z"),
  "state" : "WA", "price" : 43, "quantity" : 134, "shiftQuantityForState" : 140 }
{ "_id" : 3, "type" : "vanilla", "orderDate" : ISODate("2020-02-08T13:13:23Z"),
  "state" : "WA", "price" : 13, "quantity" : 104, "shiftQuantityForState" : 134 }

后退

$setUnion

来年

$size