String expressions ($concat, $toUpper, $toLower) in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this MongoDB aggregation pipeline?

Consider the following document in a collection:

{ "firstName": "Alice", "lastName": "Smith" }

What will be the value of the field fullName after running this pipeline?

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

MongoDB

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

A "Alice SMITH"

B "ALICE Smith"

C "alice smith"

D "Alice Smith"

💡 Hint

Remember that $toUpper converts the string to uppercase letters.

The $concat operator joins the firstName, a space, and the lastName converted to uppercase. So lastName 'Smith' becomes 'SMITH'.

❓ query_result

intermediate

What does this aggregation pipeline output for the field greeting?

Given a document:

{ "name": "bob" }

Run this pipeline:

[{ $project: { greeting: { $concat: ["Hello, ", { $toUpper: { $substrCP: ["$name", 0, 1] } }, { $toLower: { $substrCP: ["$name", 1, 2] } }] } } }]

MongoDB

[{ $project: { greeting: { $concat: ["Hello, ", { $toUpper: { $substrCP: ["$name", 0, 1] } }, { $toLower: { $substrCP: ["$name", 1, 2] } }] } } }]

A "Hello, Bob"

B "Hello, BOB"

C "Hello, bob"

D "Hello, BoB"

💡 Hint

Look at how the first letter is uppercased and the rest lowercased.

The pipeline capitalizes the first letter of the name and lowercases the next two letters, resulting in 'Bob'.

📝 Syntax

advanced

Which option causes a syntax error in this MongoDB aggregation expression?

Identify the option that will cause a syntax error when used inside a $project stage:

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

MongoDB

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

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

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

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

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

💡 Hint

Check how $toUpper is wrapped inside the $concat array.

Option C is missing curly braces around $toUpper expression, causing a syntax error.

❓ optimization

advanced

Which option is the most efficient way to concatenate first and last names in uppercase?

You want to create a field fullNameUpper that combines firstName and lastName in uppercase letters.
Which pipeline stage is best?

A { $project: { fullNameUpper: { $toUpper: "$firstName" } } }

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

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

D { $project: { fullNameUpper: { $concat: [ { $toUpper: "$firstName" }, " ", { $toUpper: "$lastName" } ] } } }

💡 Hint

Applying $toUpper once after concatenation is usually more efficient.

Option B applies $toUpper once on the whole concatenated string, reducing function calls and improving performance.

🧠 Conceptual

expert

What will be the output of this aggregation pipeline for the field result?

Given a document:

{ "word": "MongoDB" }

Run this pipeline:

[{ $project: { result: { $concat: [ { $toLower: { $substrCP: ["$word", 0, 3] } }, { $toUpper: { $substrCP: ["$word", 3, 3] } }, { $toLower: { $substrCP: ["$word", 6, 2] } } ] } } }]

MongoDB

[{ $project: { result: { $concat: [ { $toLower: { $substrCP: ["$word", 0, 3] } }, { $toUpper: { $substrCP: ["$word", 3, 3] } }, { $toLower: { $substrCP: ["$word", 6, 2] } } ] } } }]

A "monGODB"

B "mongodb"

C "monGODBd"

D "monGODb"

💡 Hint

Look carefully at the substring positions and lengths.

The first 3 letters 'Mon' become 'mon', next 3 letters 'goD' become 'GOD', last 2 letters 'b' plus one extra character (empty) become 'b'. So result is 'monGODb'.