$cmp (aggregation)

Definition定义

$cmp

Compares two values and returns:比较两个值并返回：

• -1 if the first value is less than the second.如果第一值小于第二值。
• 1 if the first value is greater than the second.如果第一值大于第二值。
• 0 if the two values are equivalent.如果这两个值相等。

The $cmp compares both value and type, using the specified BSON comparison order for values of different types.$cmp对不同类型的值使用指定的BSON比较顺序来比较值和类型。

$cmp has the following syntax:具有以下语法：

{ $cmp: [ <expression1>, <expression2> ] }

For more information on expressions, see Expressions.有关表达式的详细信息，请参阅表达式。

Example实例

Consider an inventory collection with the following documents:考虑一个包含以下文档的inventory集合：

{ "_id" : 1, "item" : "abc1", description: "product 1", qty: 300 }
{ "_id" : 2, "item" : "abc2", description: "product 2", qty: 200 }
{ "_id" : 3, "item" : "xyz1", description: "product 3", qty: 250 }
{ "_id" : 4, "item" : "VWZ1", description: "product 4", qty: 300 }
{ "_id" : 5, "item" : "VWZ2", description: "product 5", qty: 180 }

The following operation uses the $cmp operator to compare the qty value with 250:以下操作使用$cmp运算符将qty值与250进行比较：

db.inventory.aggregate(
   [
     {
       $project:
          {
            item: 1,
            qty: 1,
            cmpTo250: { $cmp: [ "$qty", 250 ] },
            _id: 0
          }
     }
   ]
)

The operation returns the following results:该操作返回以下结果：

{ "item" : "abc1", "qty" : 300, "cmpTo250" : 1 }
{ "item" : "abc2", "qty" : 200, "cmpTo250" : -1 }
{ "item" : "xyz1", "qty" : 250, "cmpTo250" : 0 }
{ "item" : "VWZ1", "qty" : 300, "cmpTo250" : 1 }
{ "item" : "VWZ2", "qty" : 180, "cmpTo250" : -1 }