MongoDB $floor 运算符

MongoDB 提供了各种算术表达式运算符，$floor 运算符就是其中之一。$floor 运算符用于返回小于或等于指定数字的最大整数。

语法

{ $floor:  }

重要要点：

1. 如果数字为null，则$floor运算符返回null。
2. 如果输入值引用了一个缺失的字段，则$floor运算符返回null。
3. 如果数字为NaN，则$floor运算符返回NaN。

示例

在以下示例中，我们使用的是：

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

{
        {
                "_id" : ObjectId("56254d4fdf2222265r4g12ds3d65f"),
                "std_name" : "Tin",
                "class" : "XI",   
                "gender" : "Female",
                "Father_name" : "Jonny", 
                "fees" : 5000,
                "exam_fees" : 500,
                "age" : 17,
                "grade" : 9.6,
                "Result" : "Pass"
        },
        {
                "_id" : ObjectId("56254d4fdf2222265r4g12ds37412"),
                "std_name" : "Olly",
                "class" : "X",   
                "gender" : "Female",
                "Father_name" : "Mike", 
                "fees" : 6000,
                "exam_fees" : 500,
                "age" : 16,
                "grade" : 8.4,
                "Result" : "Pass"
        },
        {
                "_id" : ObjectId("56254d4fdf2222265r4g12ds3d636"),
                "std_name" : "John",
                "class" : "XII",   
                "gender" : "Male",
                "Father_name" : "Jin", 
                "fees" : 10000,
                "exam_fees" : 500,
                "age" : 18,
                "grade" : 5.6
        },
        {
                "_id" : ObjectId("56254d4fdf2222265r4g121235489"),
                "std_name" : "Oliva",
                "class" : "VII",   
                "gender" : "Female",
                "Father_name" : "kinny", 
                "fees" : 3000,
                "exam_fees" : 500,
                "age" : 14,
                "grade" : 9.7,
                "Result" : "Pass"
        },
        {
                "_id" : ObjectId("56254d4fdf2222265r4g121235652"),
                "std_name" : "Marry",
                "class" : "VIII",   
                "gender" : "Female",
                "Father_name" : "Jonny", 
                "fees" : null,
                "exam_fees" : 500,
                "age" : 15,
                "grade" : 6.1,
                "Result" : "Pass"
        },
        {
                "_id" : ObjectId("56254d4fdf2222265r4g121236456"),
                "std_name" : "Paul",
                "class" : "IX",   
                "gender" : "Male",
                "Father_name" : "Kane", 
                "fees" : 5000,
                "exam_fees" : 500,
                "age" : 16,
                "grade" : {
                                  "internal" : 4.5,
                                  "external" : 7.4,     
                                }
                "Result" : "Pass"
        },
}

示例1：使用$floor运算符

在这个示例中，我们将会计算女学生的成绩字段的小于或等于该字段的最大整数。

db.student.aggregate(
 [
   {match: {gender : "Female"}},
      {project: 
              {
                std_name : 1,
                class : 1,
                grade : 1,
                floor_grade: {floor: "grade"}
               }
      }
 ]
)

输出：

{
                "_id" : ObjectId("56254d4fdf2222265r4g12ds3d65f"),
                "std_name" : "Tin",
                "class" : "XI",   
                "grade" : 9.6,
                "floor_grade" : "9"
},
{
                "_id" : ObjectId("56254d4fdf2222265r4g12ds37412"),
                "std_name" : "Olly",
                "class" : "X",   
                "grade" : 8.4,
                "floor_grade" : "8"
 },
 {
                "_id" : ObjectId("56254d4fdf2222265r4g121235489"),
                "std_name" : "Oliva",
                "class" : "VII",   
                "grade" : 9.7,
                "floor_grade" : "9"
 },
 {
                "_id" : ObjectId("56254d4fdf2222265r4g121235652"),
                "std_name" : "Marry",
                "class" : "VIII",   
                "grade" : 6.1,
                "floor_grade" : "6"
 },

示例2：缺少字段

在这个示例中，我们将$floor运算符应用于”class XII”学生的”phone_no”字段。

db.student.aggregate(
 [
   {match: {class : "XII"}},
      {project: 
              {
                std_name : 1,
                floor_phone: {floor: "phone_no"}
               }
      }
 ]
)

输出:

{
                "_id" : ObjectId("56254d4fdf2222265r4g12ds3d636"),
                "std_name" : "John",
                "floor_phone" : null
 },

示例3：Null值

在这个示例中，我们将$floor操作符应用于“VIII年级”学生的“费用”字段。

db.student.aggregate(
 [
   {match: {class : "VIII"}},
      {project: 
              {
                std_name : 1,
                floor_fees: {floor: "fees"}
               }
      }
 ]
)

输出：

{
                "_id" : ObjectId("56254d4fdf2222265r4g12ds3d636"),
                "std_name" : "Marry",
                "floor_fees" : null
 },

示例4：MongoDB $floor运算符（检索嵌入文档中的数据）

在这个示例中，我们要找到“IX班级”学生的grade.internal字段中小于或等于最大整数的值。

db.student.aggregate(
 [
   {match: {class : "IX"}},
      {project: 
              {
                std_name : 1,
                class : 1,
                grade.internal : 1,
                floor_grade: {floor: "grade.internal"}
               }
      }
 ]
)

输出：

{
             "_id" : ObjectId("56254d4fdf2222265r4g121236456"),
             "std_name" : "Paul",
             "class" : "IX",   
             "grade" : {
                          "internal" : 4.5
                             }
             "floor_grade" : 4
}