$mul operator for multiplication in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to multiply the field 'price' by 2 in the update operation.

MongoDB

db.products.updateOne({ _id: 1 }, { $mul: { price: [1] } })

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

A2

Bprice

C1

D$price

Attempts:

3 left

💡 Hint

Common Mistakes

Using the field name instead of a number as multiplier.

Including a dollar sign before the multiplier number.

The $mul operator multiplies the value of the specified field by the given number. Here, multiplying 'price' by 2 doubles its value.

2fill in blank

medium

Complete the code to multiply the 'quantity' field by 5 for all documents matching the filter.

MongoDB

db.inventory.updateMany({ category: 'books' }, { $mul: { quantity: [1] } })

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

A5

Bcategory

C'5'

D$quantity

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string '5' instead of number 5.

Using the field name or a dollar sign before the multiplier.

The $mul operator requires a number to multiply the field. Here, 5 is the correct multiplier.

3fill in blank

hard

Fix the error in the update command to multiply 'score' by 3.

MongoDB

db.scores.updateOne({ user: 'alice' }, { $mul: { score: [1] } })

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

Ascore

B3

C$3

D'3'

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string '3' instead of number 3.

Using the field name or dollar sign as multiplier.

The multiplier must be a number without quotes or dollar sign. Using 3 correctly multiplies the 'score' field by 3.

4fill in blank

hard

Fill both blanks to multiply 'price' by 1.5 and 'stock' by 10 in a single update.

MongoDB

db.store.updateOne({ _id: 10 }, { $mul: { price: [1], stock: [2] } })

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

A1.5

B10

C'10'

D'1.5'

Attempts:

3 left

💡 Hint

Common Mistakes

Using strings instead of numbers for multipliers.

Mixing up the values for 'price' and 'stock'.

Both multipliers must be numbers without quotes. 1.5 multiplies 'price' and 10 multiplies 'stock'.

5fill in blank

hard

Fill all three blanks to multiply 'rating' by 2, 'views' by 100, and 'likes' by 5 in one update.

MongoDB

db.analytics.updateMany({ active: true }, { $mul: { rating: [1], views: [2], likes: [3] } })

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

A2

B100

C5

D'5'

Attempts:

3 left

💡 Hint

Common Mistakes

Using strings instead of numbers for multipliers.

Using dollar signs or field names instead of numbers.

All multipliers must be numbers without quotes. 2 for 'rating', 100 for 'views', and 5 for 'likes' correctly apply the multiplication.

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