MongoDB $cmp 运算符

MongoDB $cmp 运算符 介绍

MongoDB 提供了多种聚合管道操作符。$cmp 运算符就是这些运算符之一。$cmp 运算符用于比较两个值或类型并返回结果。

它根据这些条件返回以下结果：

• 如果两个值相等，则返回 0。
• 如果第一个值大于第二个值，则返回 1。
• 如果第一个值小于第二个值，则返回 -1。

MongoDB $cmp 运算符 语法

{ $cmp : [ < expression 1 >, < expression 2 > ] }  

MongoDB $cmp 运算符 例子

假设我们有一个包含以下文档的产品集合。

{  
        {  
         "_id" : 1,   
         "name" : "rectangle",  
         "length" : 11,  
         "breath" : 10  
        }  
        {  
         "_id" : 2,   
         "name" : "square",  
         "length" : 10,  
         "breath" : 10    
        }  
        {  
         "_id" : 3,   
         "name" : "rectangle",  
         "length" : 14,  
         "breath" : 16    
        }  
        {  
         "_id" : 4,   
         "name" : "square",  
         "length" : 6,  
         "breath" : 6  
        }  
        {  
         "_id" : 5,   
         "name" : "rectangle",  
         "length" : 11,  
         "breath" : 16  
        }  
}  

示例 1：使用 $cmp 运算符

在此示例中，我们将使用 $cmp 运算符比较长度和宽度字段。

db.products.aggregate (  
  [  
    { $project :   
           {   
               _id : 1,  
               name : 1,  
               length : 1,  
               breadth : 1,  
               result: { $cmp : [ "$length", "$breadth" ] }  
           }  
    }  
  ]  
)  

输出结果为：

{
         "_id" : 1, 
         "name" : "rectangle",
         "length" : 11,
         "breath" : 10,
         "result" : 1
}
{
         "_id" : 2, 
         "name" : "square",
         "length" : 10,
         "breath" : 10,
         "result" : 0
}
{
         "_id" : 3, 
         "name" : "rectangle",
         "length" : 14,
         "breath" : 16,
         "result" : -1
}
{
         "_id" : 4, 
         "name" : "square",
         "length" : 6,
         "breath" : 6,
         "result" : 0
}
{
         "_id" : 5, 
         "name" : "rectangle",
         "length" : 11,
         "breath" : 16,
         "result" : -1
}   

假设我们将以下文档添加到我们的集合中：

{  
        {  
         "_id" : 6,   
         "name" : "rectangle",  
         "length" : null,  
         "breath" : 10  
        }  
        {  
         "_id" : 7,   
         "name" : "square",  
         "length" : 10,  
         "breath" : null      
        }  
        {  
         "_id" : 8,   
         "name" : "rectangle",  
         "length" : null,  
         "breath" : null      
        }  
}  

示例 2：比较NULL值

db.products.aggregate (  
  [  
    { $match : {_id : { $nin : [ 1, 2, 3, 4, 5 ] } } },   
       { $project :   
           {   
               _id : 1,  
               name : 1,  
               length : 1,  
               breadth : 1,  
               result: { $cmp : [ "$length", "$breadth" ] }  
           }  
    }  
  ]  
)  

输出结果为：

{
         "_id" : 6, 
         "name" : "rectangle",
         "length" : null,
         "breath" : 10,
         "result" : -1
}
{
         "_id" : 7, 
         "name" : "square",
         "length" : 10,
         "breath" : null,
         "result" : 1
}
{
         "_id" : 8, 
         "name" : "rectangle",
         "length" : null,
         "breath" : null,
         "result" : 0
}  

假设我们将以下文档添加到我们的集合中：

{  
         "_id" : 9,   
         "name" : "item_1",  
         "a" : ISODate("2020-05-09T23:30:15.100Z"),  
         "b" : 10  
}  
{  
         "_id" : 10,   
         "name" : "item_2",  
         "a" : 10,  
         "b" : ISODate("2012-11-18T04:00:20.112Z")    
}  
{  
         "_id" : 11,   
         "name" : "item_3",  
         "a" : ISODate("2018-11-14T04:00:20.112Z"),  
         "b" : ISODate("2015-10-07T04:00:20.112Z")    
}  

示例 3：比较不同类型

db.products.aggregate (  
  [  
    { $match : {_id : { $nin : [ 1, 2, 3, 4, 5, 6, 7, 8 ] } } },   
       { $project :   
           {   
               _id : 1,  
               name : 1,  
               a : 1,  
               b : 1,  
               result: { $cmp : [ "$a", "$b" ] }  
           }  
    }  
  ]  
)  

输出结果为：

{
         "_id" : 9, 
         "name" : "item_1",
         "a" : ISODate("2020-05-09T23:30:15.100Z"),
         "b" : 10,
         "result" : 1
}
{
         "_id" : 10, 
         "name" : "item_2",
         "a" : 10,
         "b" : ISODate("2012-11-18T04:00:20.112Z"),	
         "result" : -1
}
{
         "_id" : 11, 
         "name" : "item_3",
         "a" : ISODate("2018-11-14T04:00:20.112Z"),
         "b" : ISODate("2015-10-07T04:00:20.112Z"),
         "result" : 1
}