Arithmetic expressions ($add, $multiply, $divide) in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this MongoDB aggregation?

Given a collection with documents like {"value1": 10, "value2": 5}, what will be the result of this aggregation pipeline stage?

{ $project: { result: { $add: ["$value1", "$value2"] } } }

MongoDB

{ $project: { result: { $add: ["$value1", "$value2"] } } }

A [{ "result": 5 }]

B [{ "result": 50 }]

C [{ "result": 15 }]

D [{ "result": 2 }]

💡 Hint

Think about what $add does with two numbers.

The $add operator sums the values of value1 and value2, so 10 + 5 = 15.

❓ query_result

intermediate

What does this $multiply expression return?

For documents with fields {"a": 4, "b": 3}, what is the output of this aggregation stage?

{ $project: { product: { $multiply: ["$a", "$b"] } } }

MongoDB

{ $project: { product: { $multiply: ["$a", "$b"] } } }

A [{ "product": 0 }]

B [{ "product": 7 }]

C [{ "product": 1.33 }]

D [{ "product": 12 }]

💡 Hint

Multiply the two numbers.

4 multiplied by 3 equals 12.

📝 Syntax

advanced

Which option causes a syntax error in this $divide expression?

Identify which aggregation stage will cause a syntax error when trying to divide two fields:

{ $project: { ratio: { $divide: ["$num", "$den"] } } }

MongoDB

{ $project: { ratio: { $divide: ["$num", "$den"] } } }

A { $project: { ratio: { $divide: "$num" } } }

B { $project: { ratio: { $divide: ["$num", "$den"] } } }

C { $project: { ratio: { $divide: ["$num"] } } }

D { $project: { ratio: { $divide: ["$num", "$den", "$extra"] } } }

💡 Hint

Check the number of arguments $divide expects.

$divide requires exactly two arguments in an array. Option A provides a single string, causing a syntax error.

❓ query_result

advanced

What is the output of this nested arithmetic expression?

Given documents with {"x": 8, "y": 2, "z": 4}, what is the result of this aggregation stage?

{ $project: { result: { $add: [ { $multiply: ["$x", "$y"] }, { $divide: ["$z", 2] } ] } } }

MongoDB

{ $project: { result: { $add: [ { $multiply: ["$x", "$y"] }, { $divide: ["$z", 2] } ] } } }

A [{ "result": 18 }]

B [{ "result": 20 }]

C [{ "result": 16 }]

D [{ "result": 12 }]

💡 Hint

Calculate multiplication and division first, then add.

Multiply 8*2=16, divide 4/2=2, then add 16+2=18.

🧠 Conceptual

expert

Why does this $divide operation return null?

Consider documents with {"a": 10, "b": 0}. What is the output of this aggregation stage?

{ $project: { result: { $divide: ["$a", "$b"] } } }

Why does it return null instead of an error?

MongoDB

{ $project: { result: { $divide: ["$a", "$b"] } } }

A MongoDB throws a syntax error for division by zero.

B MongoDB returns null to avoid runtime errors when dividing by zero.

C MongoDB returns Infinity for division by zero.

D MongoDB returns zero when dividing by zero.

💡 Hint

Think about how MongoDB handles invalid math operations.

MongoDB returns null for division by zero to prevent pipeline failure.