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$add
Adds numbers together or adds numbers and a date. 将数字相加或将数字和日期相加。If one of the arguments is a date, 如果其中一个参数是日期，$add treats the other arguments as milliseconds to add to the date.$add将其他参数视为要添加到日期的毫秒。
The $add expression has the following syntax:$add表达式语法如下：
{ $add: [ <expression1>, <expression2>, ... ] }
The arguments can be any valid expression as long as they resolve to either all numbers or to numbers and a date. 参数可以是任何有效的表达式，只要它们解析为所有数字或数字和日期。For more information on expressions, see Expressions.有关表达式的详细信息，请参阅表达式。
The following examples use a 以下示例使用具有以下文档的sales collection with the following documents:sales集合：
{ "_id" : 1, "item" : "abc", "price" : 10, "fee" : 2, date: ISODate("2014-03-01T08:00:00Z") }
{ "_id" : 2, "item" : "jkl", "price" : 20, "fee" : 1, date: ISODate("2014-03-01T09:00:00Z") }
{ "_id" : 3, "item" : "xyz", "price" : 5, "fee" : 0, date: ISODate("2014-03-15T09:00:00Z") }
The following aggregation uses the 以下聚合使用$add expression in the $project pipeline to calculate the total cost:$project管道中的$add表达式计算总成本：
db.sales.aggregate(
[
{ $project: { item: 1, total: { $add: [ "$price", "$fee" ] } } }
]
)
The operation returns the following results:该操作返回以下结果：
{ "_id" : 1, "item" : "abc", "total" : 12 }
{ "_id" : 2, "item" : "jkl", "total" : 21 }
{ "_id" : 3, "item" : "xyz", "total" : 5 }
The following aggregation uses the 以下聚合使用$add expression to compute the billing_date by adding 3*24*60*60000 milliseconds (i.e. 3 days) to the date field :$add表达式通过向日期字段添加3*24*60*60000毫秒（即3天）来计算billing_date：
db.sales.aggregate(
[
{ $project: { item: 1, billing_date: { $add: [ "$date", 3*24*60*60000 ] } } }
]
)
The operation returns the following results:该操作返回以下结果：
{ "_id" : 1, "item" : "abc", "billing_date" : ISODate("2014-03-04T08:00:00Z") }
{ "_id" : 2, "item" : "jkl", "billing_date" : ISODate("2014-03-04T09:00:00Z") }
{ "_id" : 3, "item" : "xyz", "billing_date" : ISODate("2014-03-18T09:00:00Z") }