$substr (aggregation)

Definition定义

$substr

Deprecated since version 3.4: $substr is now an alias for $substrBytes.从版本3.4开始不推荐使用：$substr现在是$substrBytes的别名。

Returns a substring of a string, starting at a specified index position and including the specified number of characters. 返回字符串的子字符串，从指定的索引位置开始，包括指定的字符数。The index is zero-based.索引是从零开始的。

$substr has the following syntax:语法如下：

{ $substr: [ <string>, <start>, <length> ] }

The arguments can be any valid expression as long as the first argument resolves to a string, and the second and third arguments resolve to integers. 只要第一个参数解析为字符串，第二个和第三个参数解析成整数，参数就可以是任何有效的表达式。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

If <start> is a negative number, $substr returns an empty string "".如果<start>为负数，$substr将返回空字符串""。

If <length> is a negative number, $substr returns a substring that starts at the specified index and includes the rest of the string.如果<length>是负数，$substr将返回一个子字符串，该子字符串从指定的索引开始，并包含字符串的其余部分。

$substr only has a well-defined behavior for strings of ASCII characters.只对ASCII字符字符串具有定义良好的行为。

Example示例

Consider an inventory collection with the following documents:考虑使用以下文档进行库存集合：

{ "_id" : 1, "item" : "ABC1", quarter: "13Q1", "description" : "product 1" }
{ "_id" : 2, "item" : "ABC2", quarter: "13Q4", "description" : "product 2" }
{ "_id" : 3, "item" : "XYZ1", quarter: "14Q2", "description" : null }

The following operation uses the $substr operator to separate the quarter value into a yearSubstring and a quarterSubstring:以下操作使用$substr运算符将quarter值分隔为yearSubstring和quarterSubstring：

db.inventory.aggregate(
   [
     {
       $project:
          {
            item: 1,
            yearSubstring: { $substr: [ "$quarter", 0, 2 ] },
            quarterSubtring: { $substr: [ "$quarter", 2, -1 ] }
          }
      }
   ]
)

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

{ "_id" : 1, "item" : "ABC1", "yearSubstring" : "13", "quarterSubtring" : "Q1" }
{ "_id" : 2, "item" : "ABC2", "yearSubstring" : "13", "quarterSubtring" : "Q4" }
{ "_id" : 3, "item" : "XYZ1", "yearSubstring" : "14", "quarterSubtring" : "Q2" }