String expressions ($concat, $toUpper, $toLower) in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $concat expression do in MongoDB?

It joins two or more strings together into one string.

beginner

How does $toUpper transform a string in MongoDB?

It changes all letters in the string to uppercase letters.

beginner

What is the purpose of $toLower in MongoDB string expressions?

It converts all letters in a string to lowercase letters.

intermediate

Write a MongoDB expression to combine first name and last name with a space in between.

Use { $concat: ["$firstName", " ", "$lastName"] } to join firstName and lastName with a space.

intermediate

How can you convert a user's name to uppercase and then concatenate with a greeting in MongoDB?

Use { $concat: ["Hello, ", { $toUpper: "$name" }] } to greet with uppercase name.

Which MongoDB expression joins multiple strings into one?

A $toUpper

B $toLower

C $concat

D $split

$concat joins strings; $toUpper and $toLower change case; $split breaks strings apart.

What does $toUpper do to the string "hello"?

A "HELLO"

B "hello"

C "Hello"

D "hElLo"

$toUpper converts all letters to uppercase.

How would you convert "WORLD" to lowercase in MongoDB?

A { $toLower: "WORLD" }

B { $toUpper: "WORLD" }

C { $concat: ["W", "O", "R", "L", "D"] }

D { $substr: ["WORLD", 0, 5] }

$toLower changes all letters to lowercase.

Which expression correctly joins firstName and lastName with a space?

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

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

C { $toUpper: ["$firstName", " ", "$lastName"] }

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

You need to add a space string " " between the two fields in $concat.

What will this expression return? { $concat: ["Hi, ", { $toUpper: "$user" }] } if user is "anna"?

A "Hi, anna"

B "hi, ANNA"

C "ANNA"

D "Hi, ANNA"

It concatenates "Hi, " with the uppercase version of "anna", which is "ANNA".

Explain how to use $concat, $toUpper, and $toLower in MongoDB to format a full name.

Think about joining first and last names and changing case.

Describe a real-life example where $concat and $toUpper could be used together in a MongoDB query.

Imagine saying hello to a user with their name in capital letters.

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