$and operator behavior in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What documents does this $and query return?

Consider a collection students with documents like:

{ "name": "Alice", "age": 20, "grade": "A" }

What documents will this query return?

{ $and: [ { age: { $gt: 18 } }, { grade: "A" } ] }

MongoDB

{ $and: [ { age: { $gt: 18 } }, { grade: "A" } ] }

A Documents where age is greater than 18 or grade is 'A'

B Documents where age is greater than 18 and grade is 'A'

C Documents where age is less than or equal to 18 and grade is 'A'

D Documents where age is greater than 18 but grade is not 'A'

💡 Hint

Remember $and requires all conditions to be true for a document.

The $and operator returns documents that satisfy all conditions inside the array. Here, both age > 18 and grade = 'A' must be true.

📝 Syntax

intermediate

Which $and query syntax is correct?

Choose the syntactically valid MongoDB query using $and to find documents with age 25 and city 'NY'.

A { $and: ( { age: 25 }, { city: 'NY' } ) }

B { $and: { age: 25, city: 'NY' } }

C { $and: [ age: 25, city: 'NY' ] }

D { $and: [ { age: 25 }, { city: 'NY' } ] }

💡 Hint

Check the data structure used with $and: it must be an array of conditions.

The $and operator requires an array of condition objects. Option D correctly uses an array with two objects.

❓ optimization

advanced

Which query is more efficient to find documents with age 30 and status 'active'?

Given a collection with indexes on age and status, which query is better for performance?

A { $or: [ { age: 30 }, { status: 'active' } ] }

B { $and: [ { age: 30 }, { status: 'active' } ] }

C { age: 30, status: 'active' }

D { $and: { age: 30, status: 'active' } }

💡 Hint

MongoDB treats multiple fields in a query as an implicit AND.

Specifying multiple fields directly is equivalent to $and and is more concise and optimized by MongoDB.

🔧 Debug

advanced

Why does this $and query return no results?

Given documents with fields 'score' and 'passed', why does this query return no documents?

{ $and: [ { score: { $gt: 50 } }, { score: { $lt: 40 } } ] }

MongoDB

{ $and: [ { score: { $gt: 50 } }, { score: { $lt: 40 } } ] }

A No document can have score greater than 50 and less than 40 at the same time

B The syntax of $and is incorrect

C The fields 'score' and 'passed' are missing in documents

D The query should use $or instead of $and

💡 Hint

Think about the logic of the conditions combined with $and.

The conditions contradict each other: score cannot be both greater than 50 and less than 40 simultaneously.

🧠 Conceptual

expert

How does MongoDB internally treat multiple conditions combined with $and?

Which statement best describes how MongoDB processes queries with $and operator?

A MongoDB evaluates each condition separately and returns documents that satisfy all conditions, using indexes if available

B MongoDB converts $and queries into $or queries internally for optimization

C MongoDB only uses the first condition in the $and array and ignores the rest

D MongoDB merges all conditions into a single condition and evaluates it once

💡 Hint

Think about how multiple filters work together and how indexes help.

MongoDB evaluates each condition in $and and combines results, using indexes to optimize each condition's evaluation.