在 MongoDB 中串联字段 (Concatenate Fields) 使用 $concat

本文演示如何在 MongoDB Aggregation Pipeline 中使用 $concat 操作符将多个字符串字段连接成一个新字段。

创建示例数据

> db.concatenateFieldsDemo.insertOne({"StudentFirstName":"Adam","StudentLastName":"Smith"});
{ "acknowledged" : true, "insertedId" : ObjectId("5cd6ebf46d78f205348bc62e") }
> db.concatenateFieldsDemo.insertOne({"StudentFirstName":"John","StudentLastName":"Doe"});
{ "acknowledged" : true, "insertedId" : ObjectId("5cd6ebfc6d78f205348bc62f") }
> db.concatenateFieldsDemo.insertOne({"StudentFirstName":"David","StudentLastName":"Miller"});
{ "acknowledged" : true, "insertedId" : ObjectId("5cd6ec376d78f205348bc630") }
> db.concatenateFieldsDemo.insertOne({"StudentFirstName":"Sam","StudentLastName":"Williams"});
{ "acknowledged" : true, "insertedId" : ObjectId("5cd6ec436d78f205348bc631") }

显示原始数据

> db.concatenateFieldsDemo.find().pretty();

{ "_id" : ObjectId("5cd6ebf46d78f205348bc62e"), "StudentFirstName" : "Adam", "StudentLastName" : "Smith" }
{ "_id" : ObjectId("5cd6ebfc6d78f205348bc62f"), "StudentFirstName" : "John", "StudentLastName" : "Doe" }
{ "_id" : ObjectId("5cd6ec376d78f205348bc630"), "StudentFirstName" : "David", "StudentLastName" : "Miller" }
{ "_id" : ObjectId("5cd6ec436d78f205348bc631"), "StudentFirstName" : "Sam", "StudentLastName" : "Williams" }

使用 $concat 连接 FirstName 和 LastName（中间加 / 分隔符）

> db.concatenateFieldsDemo.aggregate([
  { $project: {
    "StudentFullName": { $concat: ["$StudentFirstName", "/", "$StudentLastName"] },
    "StudentFirstName": 1,
    "StudentLastName": 1
  }}
]);

结果

{ "_id" : ObjectId("5cd6ebf46d78f205348bc62e"), "StudentFirstName" : "Adam", "StudentLastName" : "Smith", "StudentFullName" : "Adam/Smith" }
{ "_id" : ObjectId("5cd6ebfc6d78f205348bc62f"), "StudentFirstName" : "John", "StudentLastName" : "Doe", "StudentFullName" : "John/Doe" }
{ "_id" : ObjectId("5cd6ec376d78f205348bc630"), "StudentFirstName" : "David", "StudentLastName" : "Miller", "StudentFullName" : "David/Miller" }
{ "_id" : ObjectId("5cd6ec436d78f205348bc631"), "StudentFirstName" : "Sam", "StudentLastName" : "Williams", "StudentFullName" : "Sam/Williams" }

$concat 语法说明

• { $concat: [ <expression1>, <expression2>, ... ] }
• 所有参数必须是字符串（或可转换为字符串）
• 如果任一参数为 null/missing，结果为 null（除非用 $ifNull 处理）

处理可能为 Null 的字段

db.collection.aggregate([
  { $project: {
    fullName: {
      $concat: [
        { $ifNull: ["$firstName", ""] },
        " ",
        { $ifNull: ["$lastName", ""] }
      ]
    }
  }}
]);

其他字符串操作符

实际应用场景

• 生成显示用的全名、地址拼接
• 创建搜索用的组合字段（如 firstName + lastName + email）
• 构建 URL、文件路径、唯一标识符
• 报表中的格式化输出

排除 _id 在 MongoDB Aggregation 不包含其他字段

MongoDB 查询：Array 所有元素都小于指定值 ($not + $gt)