String expressions ($concat, $toUpper, $toLower) in MongoDB

Introduction

String expressions help you join words or change their case in your data. This makes text easier to read or compare.

You want to join a first name and last name into a full name.

You need to make all letters uppercase for consistency.

You want to convert text to lowercase before searching.

You want to create a greeting message by joining words.

You want to standardize email addresses by making them lowercase.

Syntax

MongoDB

1. {$concat: [string1, string2, ...]}
2. {$toUpper: string}
3. {$toLower: string}

$concat joins multiple strings into one.

$toUpper changes all letters to uppercase.

$toLower changes all letters to lowercase.

Examples

Joins "Hello", a space, and "World" to make "Hello World".

MongoDB

{ $concat: ["Hello", " ", "World"] }

Changes "hello" to "HELLO".

MongoDB

{ $toUpper: "hello" }

Changes "WORLD" to "world".

MongoDB

{ $toLower: "WORLD" }

Sample Program

This query creates three new fields for each user: fullName joins first and last names, shoutName makes the first name uppercase, and whisperName makes the last name lowercase.

MongoDB

db.users.aggregate([
  {
    $project: {
      fullName: { $concat: ["$firstName", " ", "$lastName"] },
      shoutName: { $toUpper: "$firstName" },
      whisperName: { $toLower: "$lastName" }
    }
  }
])

Important Notes

Make sure the fields you use exist and contain strings to avoid errors.

$concat can join more than two strings, including spaces or symbols.

$toUpper and $toLower only affect letters; numbers and symbols stay the same.

Summary

$concat joins strings together.

$toUpper changes text to uppercase.

$toLower changes text to lowercase.