$dateTrunc (aggregation operator)（聚合运算符）

binSize and 和unit Fields字段

Together, binSize and unit specify the time period used in the $dateTrunc calculation.binSize和unit一起指定$dateTrunc计算中使用的时间段。

For example:例如：

• If binSize is 1 and unit is hour, the time period is one hour. For the date 2021-03-20T11:30:05Z, $dateTrunc returns 2021-03-20T11:00:00Z.如果binSize为1，unit为hour，则时间段为一小时。对于date 2021-03-20T11:30:05Z，$dateTrunc返回2021-03-20T11:00:00Z。
• If binSize is 2 and unit is hour, the time period is two hours. For the date 2021-03-20T11:30:05Z, $dateTrunc returns 2021-03-20T10:00:00Z.如果binSize为2，unit为hour，则时间段为两小时。对于date 2021-03-20T11:30:05Z，$dateTrunc返回2021-03-20T10:00:00Z。

$dateTrunc:

• Divides the time for the $dateTrunc calculation into binSize time periods in the specified time unit.将$dateTrunc计算的时间划分为指定时间unit的binSize时间段。

  The time periods start at a reference date, which is determined by unit. If unit is:时间段从参考日期开始，参考日期由unit确定。如果unit为：

  • A string other than week, $dateTrunc uses a reference date of 2000-01-01T00:00:00.00Z. For example, if binSize is 10 and unit is year, example time periods are:除week之外的字符串$dateTrunc使用的参考日期为2000-01-01T00:00:00.00Z。例如，如果binSize为10，unit为year，则示例时间段为：

    • 2000-01-01T00:00:00.00Z
    • 2010-01-01T00:00:00.00Z
    • 2020-01-01T00:00:00.00Z
  • Equal to week, $dateTrunc uses a reference date that is set to the earliest first day of the week that is greater than or equal to 2000-01-01. The first day is set using startOfWeek (the default is Sunday).等于week，$dateTrunc使用的参考日期设置为大于或等于2000-01-01的一周中最早的第一天。使用startOfWeek设置第一天（默认值为星期日）。
• Returns the lower boundary of the time period that the date is in. The boundary is returned as an ISODate. 返回date所在时间段的下限。该边界以ISODate形式返回。If the binSize field is 1, $dateTrunc sets the least significant parts (as determined by unit) of the returned ISODate to 0 and keeps the rest of the ISODate the same.如果binSize字段为1，$dateTrunc会将返回的ISODate的最低有效部分（由unit确定）设置为0，并保持ISODate的其余部分不变。

If unit is:如果unit为：

• year: $dateTrunc returns the ISODate for the start of January 1 for the year in date.：$dateTrunc返回date中年份从1月1日开始的ISODate。

• quarter: $dateTrunc returns the ISODate for the start of the first day of the calendar quarter in date.：$dateTrunc返回date中日历季度第一天开始的ISODate。

  The quarters are:季度是：

  • January to March1月至3月
  • April to June4月至6月
  • July to September7月到9月
  • October to December10月至12月
• month: $dateTrunc returns the ISODate for the start of the first day of the month in date.：$dateTrunc返回date中月份第一天开始的ISODate。
• week: $dateTrunc returns the ISODate for the start of the startOfWeek day in date. The default for startOfWeek is Sunday.：$dateTrunc返回date中startOfWeek日开始的ISODate。startOfWeek的默认值是星期日。
• day: $dateTrunc returns the ISODate for the start of the day in date.：$dateTrunc返回date中当天开始的ISODate。
• hour: $dateTrunc returns the ISODate for the start of the hour in date.：$dateTrunc返回date中小时开始的ISODate。
• minute: $dateTrunc returns the ISODate for the start of the minute in date.：$dateTrunc返回date中分钟开始的ISODate。
• second: $dateTrunc returns the ISODate for start of the second in date.：$dateTrunc返回date中第二个开始的ISODate。

unit and 和startOfWeek Fields字段

If unit is:如果unit为：

• A string other than week, startOfWeek is ignored.除week之外的字符串，startOfWeek将被忽略。

• Equal to week and startOfWeek is:等于week，startOfWeek为：

  • Specified: $dateTrunc uses startOfWeek as the first day of the week for the calculation.指定：$dateTrunc使用startOfWeek作为一周的第一天进行计算。
  • Omitted: $dateTrunc uses Sunday as the start of the week for the calculation.省略：$dateTrunc使用周日作为一周的开始进行计算。

Truncate Order Dates in a $project Pipeline Stage截断$project管道阶段的订单日期

This example uses $dateTrunc in a $project stage to truncate the cake sales orderDate values to two weeks:此示例在$project阶段使用$dateTrunc将蛋糕销售订单日期值截断为两周：

db.cakeSales.aggregate( [
   {
      $project: {
         _id: 1,
         orderDate: 1,
         truncatedOrderDate: {
            $dateTrunc: {
               date: "$orderDate", unit: "week", binSize: 2,
               timezone: "America/Los_Angeles", startOfWeek: "Monday"
            }
         }
      }
   }
] )

In the example:在示例中：

• $project includes the _id, orderDate, and truncatedOrderDate fields in the output.输出中包括_id、orderDate和truncatedOrderDate字段。

• $dateTrunc truncates the orderDate field to a 2 binSize week unit time period in the America/Los_Angeles timezone with startOfWeek set to Monday.$dateTrunc将orderDate字段截断为America/Los_Angeles时区中的2 binSize周单位时间段，startOfWeek设置为Monday。

In this example output, the truncated orderDate is shown in the truncatedOrderDate field:在此示例输出中，截断的orderDate显示在截断的orderDate字段中：

[
   {
      _id: 0,
      orderDate: ISODate("2020-05-18T14:10:30.000Z"),
      truncatedOrderDate: ISODate("2020-05-11T07:00:00.000Z")
   },
   {
      _id: 1,
      orderDate: ISODate("2021-03-20T11:30:05.000Z"),
      truncatedOrderDate: ISODate("2021-03-15T07:00:00.000Z")
   },
   {
      _id: 2,
      orderDate: ISODate("2021-01-11T06:31:15.000Z"),
      truncatedOrderDate: ISODate("2021-01-04T08:00:00.000Z")
   },
   {
      _id: 3,
      orderDate: ISODate("2020-02-08T13:13:23.000Z"),
      truncatedOrderDate: ISODate("2020-02-03T08:00:00.000Z")
   },
   {
      _id: 4,
      orderDate: ISODate("2019-05-18T16:09:01.000Z"),
      truncatedOrderDate: ISODate("2019-05-13T07:00:00.000Z")
   },
   {
      _id: 5,
      orderDate: ISODate("2019-01-08T06:12:03.000Z"),
      truncatedOrderDate: ISODate("2019-01-07T08:00:00.000Z")
   }
]

Truncate Order Dates and Obtain Quantity Sum in a $group Pipeline Stage截断订单日期并获取$group管道阶段的数量总和

This example uses $dateTrunc in a $group stage to truncate the cake sales orderDate values to six months and return the sum of the quantity values:此示例在$group阶段中使用$dateTrunc将蛋糕销售orderDate值截断为六个月，并返回数量值的总和：

db.cakeSales.aggregate( [
   {
      $group: {
         _id: {
            truncatedOrderDate: {
               $dateTrunc: {
                  date: "$orderDate", unit: "month", binSize: 6
               }
            }
         },
         sumQuantity: { $sum: "$quantity" }
      }
   }
] )

In the example:在示例中：

• $group has the _id field set to the truncatedOrderDate field to group the cakeSales documents, and returns the sum of the quantity values for each group using $sum.$group将_id字段设置为truncatedOrderDate字段，以对cakeSales文档进行分组，并使用$sum返回每个组的quantity值之和。

• $dateTrunc truncates the orderDate field to a 6 binSize month unit time period.将orderDate字段截断为6 binSize月单位时间段。

In this example output, the truncated orderDate is shown in the truncatedOrderDate field and the quantity sum is shown in the sumQuantity field:在此示例输出中，截断的orderDate显示在truncatedOrderDate字段中，quantity总和显示在sumQuantity字段中：

[
   {
      _id: { truncatedOrderDate: ISODate("2020-01-01T00:00:00.000Z") },
      sumQuantity: 224
   },
   {
      _id: { truncatedOrderDate: ISODate("2021-01-01T00:00:00.000Z") },
      sumQuantity: 285
   },
   {
      _id: { truncatedOrderDate: ISODate("2019-01-01T00:00:00.000Z") },
      sumQuantity: 296
   }
]