$mul operator for multiplication in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $mul operator do in MongoDB?

The $mul operator multiplies the value of a field by a specified number and updates the field with the new value.

beginner

How do you use $mul in an update query?

You use the $mul update operator to multiply a field's value. For example: { $mul: { fieldName: 2 } } doubles the value of fieldName.

intermediate

Can $mul be used to multiply by decimal numbers?

Yes, $mul can multiply a field by any numeric value, including decimals (floats).

intermediate

What happens if the field used with $mul does not exist in the document?

If the field does not exist, MongoDB will not create it or perform multiplication. The update will not change that document.

beginner

Write a MongoDB update query using $mul to triple the value of the field price in documents where category is 'books'.

db.collection.updateMany({ category: 'books' }, { $mul: { price: 3 } })

What does the $mul operator do in MongoDB?

A Multiplies a field's value by a number

B Adds a number to a field's value

C Deletes a field from a document

D Creates a new field with a value

$mul multiplies the value of a field by a specified number.

If you want to double the value of a field score, which update operator would you use?

A { $set: { score: 2 } }

B { $inc: { score: 2 } }

C { $mul: { score: 2 } }

D { $push: { score: 2 } }

$mul multiplies the field value; doubling means multiplying by 2.

What happens if the field you want to multiply with $mul does not exist?

A MongoDB does not change the document

B MongoDB creates the field and sets it to the multiplier

C MongoDB creates the field and sets it to zero

D MongoDB throws an error

MongoDB does not create the field or change the document if the field is missing.

Can $mul multiply a field by a decimal number?

A No

B Yes

C Only if the field is an integer

D Only if the field is a string

$mul works with any numeric value, including decimals.

Which MongoDB method is commonly used with $mul to update multiple documents?

A deleteMany()

B find()

C insertOne()

D updateMany()

updateMany() updates multiple documents and can use $mul.

Explain how the $mul operator works in MongoDB and give an example of its use.

Think about how you would increase a price by multiplying it.

What happens if you try to use $mul on a field that does not exist in a document?

Consider how MongoDB handles missing fields in updates.

Practice

1. What does the $mul operator do in MongoDB?

easy

A. It multiplies the value of a field by a specified number.

B. It adds a specified number to the value of a field.

C. It divides the value of a field by a specified number.

D. It subtracts a specified number from the value of a field.

Solution

1. Step 1: Understand the purpose of $mul

   The $mul operator is used to multiply the value of a numeric field by a given number in an update operation.

2. Step 2: Compare with other arithmetic operators

   Unlike $inc which adds, or $div which divides, $mul specifically multiplies the field value.

3. Final Answer:

   It multiplies the value of a field by a specified number. -> Option A

4. Quick Check:

   $mul = multiply [OK]

Hint: Remember: $mul means multiply field by number [OK]

Common Mistakes:

• Confusing $mul with $inc (addition)
• Thinking $mul divides instead of multiplies
• Assuming $mul works on non-numeric fields

2. Which of the following is the correct syntax to multiply the field price by 3 in a MongoDB update?

easy

A. { $mul: { price: price * 3 } }

B. { $mul: { 3: price } }

C. { $mul: { price: "3" } }

D. { $mul: { price: 3 } }

Solution

1. Step 1: Identify correct operator usage

   The $mul operator requires a field name as key and a numeric multiplier as value.

2. Step 2: Check each option's structure

   { $mul: { price: 3 } } correctly uses { $mul: { price: 3 } }. { $mul: { 3: price } } reverses key and value. { $mul: { price: "3" } } uses a string instead of number. { $mul: { price: price * 3 } } uses an invalid expression.

3. Final Answer:

   { $mul: { price: 3 } } -> Option D

4. Quick Check:

   Correct syntax = { $mul: { price: 3 } } [OK]

Hint: Use field name as key and number as value inside $mul [OK]

Common Mistakes:

• Using string instead of number for multiplier
• Swapping key and value inside $mul
• Trying to use expressions inside $mul

3. Given a document { _id: 1, quantity: 5, price: 10 }, what will be the document after running db.collection.updateOne({ _id: 1 }, { $mul: { quantity: 2, price: 3 } })?

medium

A. { _id: 1, quantity: 2, price: 3 }

B. { _id: 1, quantity: 10, price: 30 }

C. { _id: 1, quantity: 5, price: 10 }

D. { _id: 1, quantity: 7, price: 13 }

Solution

1. Step 1: Understand the update operation

   The $mul operator multiplies each specified field by the given number. Here, quantity is multiplied by 2 and price by 3.

2. Step 2: Calculate new field values

   Original quantity 5 * 2 = 10, original price 10 * 3 = 30.

3. Final Answer:

   { _id: 1, quantity: 10, price: 30 } -> Option B

4. Quick Check:

   5*2=10 and 10*3=30 [OK]

Hint: Multiply each field value by given number in $mul [OK]

Common Mistakes:

• Adding instead of multiplying
• Not updating all specified fields
• Confusing field names or values

4. You run this update: db.products.updateOne({ _id: 2 }, { $mul: { stock: '2' } }). What is the issue?

medium

A. The update query is missing $set operator.

B. The field name 'stock' is invalid.

C. The multiplier '2' is a string, should be a number.

D. The filter {_id: 2} is incorrect.

Solution

1. Step 1: Check the multiplier type

   The $mul operator requires the multiplier to be a number, not a string.

2. Step 2: Identify the error in the update

   Here, '2' is a string (in quotes), which causes a type error in MongoDB.

3. Final Answer:

   The multiplier '2' is a string, should be a number. -> Option C

4. Quick Check:

   Multiplier must be numeric, not string [OK]

Hint: Multiplier must be a number, not a string [OK]

Common Mistakes:

• Using quotes around numbers in $mul
• Assuming $set is needed with $mul
• Blaming filter instead of multiplier type

5. You want to double the score field only if it is greater than 10 in documents. Which update query correctly uses $mul with a filter?

hard

A. db.collection.updateMany({ score: { $gt: 10 } }, { $mul: { score: 2 } })

B. db.collection.updateMany({}, { $mul: { score: { $gt: 10, $mul: 2 } } })

C. db.collection.updateMany({ $mul: { score: 2 } }, { score: { $gt: 10 } })

D. db.collection.updateMany({ score: 2 }, { $mul: { score: 10 } })

Solution

1. Step 1: Understand filtering in updateMany

   The first argument is the filter to select documents where score is greater than 10.

2. Step 2: Apply $mul only to filtered documents

   The second argument uses $mul to multiply score by 2 for those documents.

3. Final Answer:

   db.collection.updateMany({ score: { $gt: 10 } }, { $mul: { score: 2 } }) -> Option A

4. Quick Check:

   Filter then multiply = db.collection.updateMany({ score: { $gt: 10 } }, { $mul: { score: 2 } }) [OK]

Hint: Filter documents first, then apply $mul in update [OK]

Common Mistakes:

• Trying to use $mul inside filter
• Swapping filter and update arguments
• Using invalid syntax mixing filter and update