MongoDB $concat 运算符

MongoDB $concat 运算符 介绍

MongoDB 提供了多种用于聚合管道阶段的字符串表达式运算符。$concat 运算符就是这些运算符之一。$concat 运算符用于将两个或多个字符串连接成一个字符串。

MongoDB $concat 运算符 语法

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

在此运算符中传递的参数可以是任何有效的表达式，直到它解析为字符串。

• 如果参数的值引用了缺失的字段或 null，则 $concat 运算符返回 null。

MongoDB $concat 运算符 例子

假设我们有一组具有以下文档的学生。

>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

在这个例子中，我们将使用 $concat 操作符连接 name 字段和 surname 字段。

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

输出结果为：

{
	"_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
}

假设我们将以下文档添加到我们的集合中：

{
	"_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

在这个例子中，我们将使用 $concat 操作符连接 name 字段和 surname 字段。

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

输出结果为：

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

示例 3：两个以上的字符串

在此示例中，我们将使用 $concat 运算符将 name 字段与 surname 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  
                        }  
    }  
  ]  
)  

输出结果为：

{
	"_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
}