MongoDB $abs 运算符

MongoDB $abs 运算符 介绍

MongoDB 提供了多种算术表达式运算符。$abs 运算符就是这些运算符之一。$abs 运算符用于返回代码中特定数字的绝对值。绝对值意味着无论数字是正数还是负数，它总是返回一个正值。

MongoDB $abs 运算符 语法

{ $abs: <number> }  

要点：

1. 如果数字为 null，则 $abs 运算符返回 null。
2. 如果输入的值引用了缺失的字段，则 $abs 运算符返回 null。
3. 如果数字是 NaN，则 $abs 运算符返回 NaN。

MongoDB $abs 运算符 例子

在以下示例中，我们正在使用

Database: Yiidian
Collection: student  
Document: Six documents that contain the details of the students  

{
        {
                "_id" : ObjectId("56254d4fdf2222265r4g12ds3d65f"),
                "std_name" : "Micky",
                "gender" : "Female",
                "class" : "X",
                "fees" : 5000,
                "exam_fees" : 500,
                "age" : 16,
                "Total_marks" : 405
                "Result" : "Pass"
        },
        {
                "_id" : ObjectId("56254d4fdf2222265r4g12ds34563"),
                "std_name" : "Moty",
                "gender" : "Male",
                "fees" : 4000,
                "exam_fees" : 500,
                "class" : "VII",
                "age" : 15,
                "Total_marks" : 705
                "Result" : "Pass"
        },
        {
                "_id" : ObjectId("56254d4fdf2222265r4g12ds31478"),
                "std_name" : "Thomas",
                "gender" : "Male",
                "fees" : 3000,
                "exam_fees" : 500,                                
                "class" : "V",
                "age" : 12,
                " Total_marks" : 450
                " Result" : "pass"
        },
        {
                "_id" : ObjectId("56254d4fdf2222265r4g12ds37832"),
                "std_name" : "Jin",
                "gender" : "Female",
                "fees" : 5000,
                "exam_fees" : 500,
                "class" : "X",
                "age" : 16,
                "Total_marks" : 750
                "Result" : "Pass"
        },
        { 
                "_id" : ObjectId("56254d4fdf2222265r4g12ds1c46"),
                "std_name" : "Mia",
                "gender" : "Female",
                "fees" : 6000,
                "exam_fees" : 500,
                "class" : "XI",
                "age" : 17,
                " Total_marks" : 450
                "Result" : "Pass"
       },
       {
                "_id" : ObjectId("56254d4fdf2222265r4g12ds315hj"),
                "std_name" : "Mike,
                "gender" : "Male",
                "fees" : {
                           "school_fees" :4000,
                           "exam_fees" : 500,
                           "pending_fees" : 950,
                         }
                "class" : "V",
                "age" : 15,
                "Total_marks" : 450
                "Result" : "Pass"
        }
}

示例 1：使用 $abs 运算符

在此示例中，我们将仅查找女学生的总费用（费用 + 考试费用）。

db.student.aggregate(  
 [  
   {$match: {gender : "Female"}},  
      {  
          $project:   
              {  
                name : 1,  
                class : 1,  
                age : 1,  
                Result : 1,   
                Total_fees: {$abs: {$add: ["$fees", "$exam_fees"]}}  
               }  
      }  
 ]  
)  

输出结果为：

{
                "_id" : ObjectId("56254d4fdf2222265r4g12ds3d65f"),
                "std_name" : "Micky",
                "class" : "X",
                "age" : 16,
                "Result" : "Pass",
                "Total_fees" : 5500
}
{
                "_id" : ObjectId("56254d4fdf2222265r4g12ds37832"),
                "std_name" : "Jin",
                "class" : "X",
                "age" : 16,
                "Result" : "Pass",
                "Total_fees" : 5500
}
{ 
                "_id" : ObjectId("56254d4fdf2222265r4g12ds1c46"),
                "std_name" : "Mia",
                "class" : "XI",
                "age" : 17,
                "Result" : "Pass",
                "Total_fees" : 6500,
}

示例 2：MongoDB $abs 运算符（检索嵌入文档中的数据）

在此示例中，我们将查找姓名为 Mike 的学生的总费用（school_fees + Exam_fees + pending_fees）。

db.student.aggregate(  
[  
   {$match : {name : "Mike"}},  
         {  
            $project :   
                    {   
                        name : 1,   
                        class : 1,  
                        age : 1,  
                        Result : 1,  
                        Total_fees : {$abs : {$add : ["$fees.school_fees",   
                                                                      "$fees.exam_fees",   
                                                                      "$fees.pending_fees" ]  
                                                         }  
                                              }  
                     }  
          }  
 ]  
)  

输出结果为：

{
  "_id" : ObjectId("56254d4fdf2222265r4g12ds315hj"),
  "std_name" : "Mike,
  "class" : "V",
  "age" : 15,
  "Result" : "Pass",
  "Total_fees" : 5450  
}