MongoDB $concat 运算符

$concat 运算符在 MongoDB 中是什么

MongoDB 提供了一系列字符串表达式操作符，用于聚合管道阶段。concat 运算符是其中一种运算符。concat 运算符用于将两个或多个字符串连接为一个字符串。

$concat 运算符的语法

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

这个运算符中传入的参数可以是任何有效的表达式，直至它解析为字符串。

• 如果参数的值引用了一个缺失的字段或null，$concat运算符将返回null。

示例

假设我们有一个学生的集合，其中包含以下文档。

>db.students.find().pretty()
{
  {
    "_id" : 1,
    "name" : "Steve",
    "surname" : "Smith",
    "department" : "B-tech",
    "fees" : 80000
  }
  {
    "_id" : 2,
    "name" : "Sandy",
    "surname" : "Beach",
    "department" : "BCA",
    "fees" : 55000
  }
  {
    "_id" : 3,
    "name" : "John",
    "surname" : "Cena",
    "department" : "MCA",
    "fees" : 85000
  }
  {
    "_id" : 4,
    "name" : "Wick",
    "surname" : "John",
    "department" : "B.com",
    "fees" : 60000
  }
  {
    "_id" : 5,
    "name" : "David",
    "surname" : "Silva",
    "department" : "null",
    "fees" : 80000
  }
}

示例1：使用$concat运算符

在这个示例中，我们将使用$concat运算符将名称字段与姓氏字段连接起来。

db.students.aggregate(
  [
    { project : {                          _id : 1,
                          name : {concat: [ "firstName", "surname" ] },
                          department : 1,
                          fees : 1
                        }
    }
  ]
)

输出：

{
    "_id" : 1,
    "name" : "SteveSmith",
    "department" : "B-tech",
    "fees" : 80000
}
{
    "_id" : 2,
    "name" : "SandyBeach",
    "department" : "BCA",
    "fees" : 55000
}
{
    "_id" : 3,
    "name" : "JohnCena",
    "department" : "MCA",
    "fees" : 85000
}
{
    "_id" : 4,
    "name" : "WickJohn",
    "department" : "B.com",
    "fees" : 60000
}
{
    "_id" : 5,
    "name" : "DavidSilva",
    "department" : "null",
    "fees" : 80000
}

假设我们将以下文件添加到我们的收藏中：

{
    "_id" : 6,
    "name" : "null",
    "surname" : "Thomas",
    "department" : "M.com",
    "fees" : 60000
}
{
    "_id" : 7,
    "name" : "David",
    "surname" : "null",
    "department" : "BCA",
    "fees" : 60000
}

示例2：Null值

在本示例中，我们将使用$concat运算符将姓名字段与姓氏字段连接起来。

db.students.aggregate(
  [
   { match : { _id : {nin: [ 1, 2, 3, 4, 5 ] } } },
      { project : {                          _id : 1,
                          name : {concat: [ "firstName", "surname" ] },
                          department : 1,
                          fees : 1
                        }
    }
  ]
)

输出：

{
    "_id" : 6,
    "name" : null,
    "department" : "M.com",
    "fees" : 60000
}
{
    "_id" : 7,
    "name" : null,
    "department" : "BCA",
    "fees" : 60000
}

示例3：超过两个字符串

在这个示例中，我们将使用$concat运算符将名字字段与姓氏连接起来。

db.students.aggregate(
  [
    { match : { _id : {nin: [ 4, 5, 6, 7 ] } } },
       { project : {                          _id : 1,
                          name : {concat: [ "firstName", " ", "surname" ] },
                          department : 1,
                          fees : 1
                        }
    }
  ]
)

输出：

{
    "_id" : 1,
    "name" : "Steve Smith",
    "department" : "B-tech",
    "fees" : 80000
}
{
    "_id" : 2,
    "name" : "Sandy Beach",
    "department" : "BCA",
    "fees" : 55000
}
{
    "_id" : 3,
    "name" : "John Cena",
    "department" : "MCA",
    "fees" : 85000
}

另一个示例：

db.students.aggregate(
  [
    { match : { _id : {nin: [ 4, 5, 6, 7 ] } } },
       { project : {                             _id : 1,
                             Student : {concat: [ "firstName", " ", "surname", ", ", "$department"] },
                             fees : 1
                           }
    }
  ]
)

输出结果：

{
    "_id" : 1,
    "Student" : "Steve Smith, B-tech",
    "fees" : 80000
}
{
    "_id" : 2,
    "Student" : "Sandy Beach, BCA",
    "fees" : 55000
}
{
    "_id" : 3,
    "Student" : "John Cena, MCA",
    "fees" : 85000
}