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.

Practice

1. What does the MongoDB expression {$toUpper: "hello"} do?

easy

A. Joins the string "hello" with another string

B. Converts the string "hello" to lowercase "hello"

C. Converts the string "hello" to uppercase "HELLO"

D. Removes spaces from the string "hello"

Solution

1. Step 1: Understand $toUpper usage

   The $toUpper operator converts all letters in a string to uppercase.

2. Step 2: Apply $toUpper to "hello" string

   Applying $toUpper to "hello" changes it to "HELLO".

3. Final Answer:

   Converts the string "hello" to uppercase "HELLO" -> Option C

4. Quick Check:

   $toUpper("hello") = "HELLO" [OK]

Hint: Remember $toUpper makes all letters uppercase [OK]

Common Mistakes:

• Confusing $toUpper with $toLower
• Thinking $toUpper joins strings
• Assuming it removes spaces

2. Which of the following is the correct syntax to concatenate two fields firstName and lastName in MongoDB aggregation?

easy

A. {$concat: "$firstName" + " " + "$lastName"}

B. {$concat: ["$firstName", " ", "$lastName"]}

C. {$concat: "$firstName", " ", "$lastName"}

D. {$concat: "$firstName $lastName"}

Solution

1. Step 1: Recall $concat syntax

   $concat requires an array of strings or field references to join them.

2. Step 2: Check each option's syntax

   {$concat: ["$firstName", " ", "$lastName"]} correctly uses an array with field references and a space string. Others misuse concatenation or array syntax.

3. Final Answer:

   {$concat: ["$firstName", " ", "$lastName"]} -> Option B

4. Quick Check:

   $concat needs array of strings [OK]

Hint: Use array syntax with $concat for multiple strings [OK]

Common Mistakes:

• Using + operator inside $concat
• Passing arguments without array brackets
• Putting fields inside a single string

3. Given the document {"name": "alice", "surname": "smith"}, what is the output of this aggregation expression?

{ $concat: [ { $toUpper: "$name" }, " ", { $toLower: "$surname" } ] }

medium

A. "Alice Smith"

B. "alice smith"

C. "ALICE SMITH"

D. "ALICE smith"

Solution

1. Step 1: Apply $toUpper to "name" field

   The "name" field is "alice", $toUpper converts it to "ALICE".

2. Step 2: Apply $toLower to "surname" field and concatenate

   The "surname" field is "smith", $toLower keeps it "smith". Then $concat joins "ALICE", space, and "smith".

3. Final Answer:

   "ALICE smith" -> Option D

4. Quick Check:

   $toUpper("alice") + " " + $toLower("smith") = "ALICE smith" [OK]

Hint: Apply $toUpper and $toLower before concatenation [OK]

Common Mistakes:

• Assuming $toLower changes uppercase to title case
• Mixing order of operations
• Forgetting to use $concat array

4. Identify the error in this MongoDB aggregation expression:

{ $concat: "$firstName", " ", "$lastName" }

medium

A. Missing array brackets around the arguments for $concat

B. Using double quotes instead of single quotes

C. Incorrect field names without $ prefix

D. Using $concat instead of $toUpper

Solution

1. Step 1: Check $concat argument format

   $concat requires a single array argument containing strings or field references.

2. Step 2: Identify missing array brackets

   The expression passes multiple arguments without wrapping them in an array, causing syntax error.

3. Final Answer:

   Missing array brackets around the arguments for $concat -> Option A

4. Quick Check:

   $concat([...]) needs array brackets [OK]

Hint: Always wrap $concat inputs in an array [OK]

Common Mistakes:

• Passing multiple arguments without array
• Confusing $concat with string concatenation operator
• Not prefixing field names with $

5. You want to create a new field fullName in your aggregation pipeline that combines firstName and lastName with a space, but both names should be in uppercase. Which expression correctly achieves this?

hard

A. {$concat: [{$toUpper: "$firstName"}, " ", {$toUpper: "$lastName"}]}

B. {$toLower: {$concat: ["$firstName", " ", "$lastName"]}}

C. {$concat: ["$firstName", " ", "$lastName"]}

D. {$toLower: {$concat: [{$toUpper: "$firstName"}, " ", {$toUpper: "$lastName"}]}}

Solution

1. Step 1: Understand requirement for uppercase names

   Both firstName and lastName must be converted to uppercase individually before concatenation.

2. Step 2: Analyze each option's logic

   {$concat: [{$toUpper: "$firstName"}, " ", {$toUpper: "$lastName"}]} applies $toUpper to each field separately, then concatenates with space. {$toLower: {$concat: ["$firstName", " ", "$lastName"]}} concatenates first, then lowercases whole string, resulting in lowercase. {$concat: ["$firstName", " ", "$lastName"]} does no uppercase. {$toLower: {$concat: [{$toUpper: "$firstName"}, " ", {$toUpper: "$lastName"}]}} converts final string to lowercase, which is wrong.

3. Final Answer:

   {$concat: [{$toUpper: "$firstName"}, " ", {$toUpper: "$lastName"}]} -> Option A

4. Quick Check:

   Uppercase each name then join with space [OK]

Hint: Uppercase each name before concat for correct result [OK]

Common Mistakes:

• Uppercasing after concatenation changes space too
• Not applying $toUpper to both fields
• Using $toLower mistakenly