String expressions ($concat, $toUpper, $toLower) in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to concatenate first and last names with a space in between.

MongoDB

{ $concat: ["$firstName", [1], "$lastName"] }

Drag options to blanks, or click blank then click option'

A" "

B"-"

C"_"

D","

Attempts:

3 left

💡 Hint

Common Mistakes

Using a dash or underscore instead of a space

Forgetting to put quotes around the space

We use a space " " to separate first and last names when concatenating.

2fill in blank

medium

Complete the code to convert the 'city' field to uppercase.

MongoDB

{ $toUpper: "$[1]" }

Drag options to blanks, or click blank then click option'

Acity

BCity

CCITY

DcitY

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase field names

Adding extra spaces inside the field name string

Field names are case-sensitive and usually lowercase; use "$city" to refer to the field.

3fill in blank

hard

Fix the error in the code to convert 'country' field to lowercase.

MongoDB

{ $toLower: "$[1]" }

Drag options to blanks, or click blank then click option'

ACountry

Bcountry

CCOUNTRY

DcOUNTRY

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase or mixed case field names

Adding spaces inside the field name string

The field name must match exactly, so use "$country" in lowercase.

4fill in blank

hard

Fill both blanks to concatenate first name and last name in uppercase with a space in between.

MongoDB

{ $concat: [ { $toUpper: "$firstName" }, [1], { $toUpper: "$lastName" } ] }

Drag options to blanks, or click blank then click option'

A","

B"-"

C"_"

D" "

Attempts:

3 left

💡 Hint

Common Mistakes

Using a dash or underscore instead of a space

Forgetting to put quotes around the space

We use a space " " to separate the uppercase first and last names when concatenating.

5fill in blank

hard

Fill all three blanks to concatenate lowercase city and uppercase country with a comma and space in between.

MongoDB

{ $concat: [ { $toLower: "$[1]" }, [2], { $toUpper: "$[3]" } ] }

Drag options to blanks, or click blank then click option'

Acity

B", "

Ccountry

D" - "

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong field names

Using dash instead of comma separator

Forgetting quotes around separator

We convert city to lowercase, country to uppercase, and separate them with a comma and space ", ".

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