$sqrt (expression operator)（表达式运算符）

Definition定义

$sqrt

Calculates the square root of a positive number and returns the result as a double.计算正数的平方根，并将结果作为双精度数返回。

$sqrt has the following syntax:具有以下语法：

{ $sqrt: <number> }

The argument can be any valid expression as long as it resolves to a non-negative number. 参数可以是任何有效的表达式，只要它解析为非负数。For more information on expressions, see Expressions.有关表达式的详细信息，请参阅表达式。

Behavior行为

The default return type is a double. If at least one operand is a decimal, then the return type is a decimal.默认返回类型是double。如果至少有一个操作数是decimal，则返回类型是decimal。

If the argument resolves to a value of null or refers to a field that is missing, $sqrt returns null. If the argument resolves to NaN, $sqrt returns NaN.如果参数解析为null值或引用缺少的字段，则$sqrt返回null。如果参数解析为NaN，$sqrt将返回NaN。

$sqrt errors on negative numbers.对负数会发生错误。

Example示例

A collection points contains the following documents:集合points包含以下文档：

db.points.insertMany( [
   { _id: 1, p1: { x: 5, y: 8 }, p2: { x: 0, y: 5} },
   { _id: 2, p1: { x: -2, y: 1 }, p2: { x: 1, y: 5} },
   { _id: 3, p1: { x: 4, y: 4 }, p2: { x: 4, y: 0} }
] )

The following example uses $sqrt to calculate the distance between p1 and p2:以下示例使用$sqrt计算p1和p2之间的距离：

db.points.aggregate([
   {
     $project: {
        distance: {
           $sqrt: {
               $add: [
                  { $pow: [ { $subtract: [ "$p2.y", "$p1.y" ] }, 2 ] },
                  { $pow: [ { $subtract: [ "$p2.x", "$p1.x" ] }, 2 ] }
               ]
           }
        }
     }
   }
])

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

{ _id: 1, distance: 5.830951894845301 }
{ _id: 2, distance: 5 }
{ _id: 3, distance: 4 }