{ "_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") }
db.sales.aggregate(
 [
 { $project: { item: 1, total: { $add: [ "$price", "$fee" ] } } }
 ]
)
{ "_id" : 2, "item" : "jkl", "total" : 21 }

{ "_id" : 3, "item" : "xyz", "total" : 5 }

有效日期 命名为 expire_date，则相应的SQL如下：
db.sales.aggregate(
 [
 { $project: { item: 1, expire_date: { $add: [ "$date", 3*24*60*60*1000 ] } } }
 ]
)
{ "_id" : 2, "item" : "jkl", "expire_date" : ISODate("2014-03-04T09:00:00Z") }

{ "_id" : 3, "item" : "xyz", "expire_date" : ISODate("2014-03-18T09:00:00Z") }
注：此案例中有关时间的这种运算，在SQL Server 数据库中是通过函数 DATEADD(）来实现的。2.$subtract操作符（－）

$subtract操作符的语法

{ "_id" : 1, "item" : "abc", "price" : 10, "fee" : 2, "discount" : 5, "date" : ISODate("2014-03-01T08:00:00Z") }

{ "_id" : 2, "item" : "jkl", "price" : 20, "fee" : 1, "discount" : 2, "date" : ISODate("2014-03-01T09:00:00Z") }

db.sales.aggregate( [ { $project: { item: 1, total: { $subtract: [ { $add: [ "$price", "$fee" ] }, "$discount" ] } } } ] )
{ "_id" : 2, "item" : "jkl", "total" : 19 }


db.sales.aggregate( [ { $project: { item: 1, dateDifference: { $subtract: [ new Date(), "$date" ] } } } ] )
{ "_id" : 2, "item" : "jkl", "dateDifference" : NumberLong("154743494624") }
注：此案例中关于时间的这种运算，在SQL Server 数据库中是通过函数 DATEDiff(）来实现的。
实现SQL的语句如下
db.sales.aggregate( [ { $project: { item: 1, dateDifference: { $subtract: [ "$date", 3 * 60 * 60 * 1000 ] } } } ] )
{ "_id" : 2, "item" : "jkl", "dateDifference" : ISODate("2014-03-01T06:00:00Z") }

3.$multiply操作符（*）


{ "_id" : 1, "item" : "abc", "price" : 10, "quantity": 2, date: ISODate("2014-03-01T08:00:00Z") }

{ "_id" : 2, "item" : "jkl", "price" : 20, "quantity": 1, date: ISODate("2014-03-01T09:00:00Z") }

{ "_id" : 3, "item" : "xyz", "price" : 5, "quantity": 10, date: ISODate("2014-03-15T09:00:00Z") }

 db.sales.aggregate(
 [
 { $project: { item: 1, total_price: { $multiply: [ "$price", "$quantity" ] } } }
 ]
)
{ "_id" : 2, "item" : "jkl", "total_price" : 20 }

{ "_id" : 3, "item" : "xyz", "total_price" : 50 }
4.$divide操作符（/）


{ "_id" : 1, "name" : "A", "hours" : 80, "resources" : 7 },

{ "_id" : 2, "name" : "B", "hours" : 40, "resources" : 4 }

db.planning.aggregate(
 [
 { $project: { name: 1, workdays: { $divide: [ "$hours", 8 ] } } }
 ]
)
{ "_id" : 2, "name" : "B", "workdays" : 5 }
5 求余 运算（$mod）

{ "_id" : 2, "project" : "B", "hours" : 40, "tasks" : 4 }

db.planning.aggregate(
 [
 { $project: { remainder: { $mod: [ "$hours", "$tasks" ] } } }
 ]
)
{ "_id" : 2, "remainder" : 0 }
6.案例分析

{ "_id" : 1, "sno" : 201501010001, "name" : "xiaoming", "attendanceAvg" : 90, "quizzAvg" : 95, "testAvg" : 98, "teacherPet" : true }
{ "_id" : 2, "sno" : 201501010002, "name" : "xiaoli", "attendanceAvg" : 100, "quizzAvg" : 90, "testAvg" : 90, "teacherPet" : false }
{ "_id" : 3, "sno" : 201501010003, "name" : "xiaohong", "attendanceAvg" : 100, "quizzAvg" : 80, "testAvg" : 100, "teacherPet" : false }
db.students.aggregate(
 [
 {
  $project:
  {

   "sno": 1, "name": 1,
   grade:
   {
    $cond: ["$teacherPet", 100, {
    "$add":
     [
     { "$multiply": ["$attendanceAvg", .1] },
     { "$multiply": ["$quizzAvg", .3] },
     { "$multiply": ["$testAvg", .6] }
     ]
    }]
   }
  }
 }
 ]
)
{ "_id" : 2, "sno" : 201501010002, "name" : "xiaoli", "grade" : 91 }

{ "_id" : 3, "sno" : 201501010003, "name" : "xiaohong", "grade" : 94 }

6.2 常用逻辑表达式