Arithmetic expressions ($add, $multiply, $divide) in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $add operator do in MongoDB aggregation?

The $add operator adds numbers or dates together. It sums the values of its arguments and returns the total.

beginner

How do you multiply two fields in a MongoDB aggregation pipeline?

Use the $multiply operator with an array of the fields or values you want to multiply. It returns the product.

intermediate

What happens if you use $divide with a divisor of zero?

MongoDB returns null because division by zero is undefined.

beginner

Write a MongoDB aggregation expression to calculate the total price by multiplying quantity and price fields.

{ $multiply: ["$quantity", "$price"] }

intermediate

Can $add be used to add dates in MongoDB? How?

Yes, $add can add a number (milliseconds) to a date to get a new date. For example, adding 86400000 (milliseconds in a day) to a date adds one day.

Which operator would you use to add two numbers in a MongoDB aggregation?

A $subtract

B $multiply

C $add

D $divide

The $add operator sums numbers or dates.

What does { $multiply: [2, 3, 4] } return?

A 9

B 24

C 6

D 14

It multiplies 2 × 3 × 4 = 24.

What is the result of { $divide: [10, 0] } in MongoDB?

A null

B 0

C Infinity

D Error

MongoDB returns null for division by zero.

How would you add 5 days to a date field startDate?

A { $add: ["$startDate", 432000000] }

B { $multiply: ["$startDate", 5] }

C { $add: ["$startDate", 5] }

D { $divide: ["$startDate", 5] }

5 days × 86400000 ms/day = 432000000 ms added to the date.

Which operator would you use to calculate the average price by dividing total price by quantity?

A $subtract

B $multiply

C $add

D $divide

Use $divide to divide total price by quantity.

Explain how to use the MongoDB $add, $multiply, and $divide operators in aggregation pipelines.

Think about simple math operations and how they apply to fields or values.

Describe a real-life example where you might use $multiply and $divide in a MongoDB aggregation.

Consider shopping or sales data.

Practice

1. What does the MongoDB expression { $add: [5, 3] } do?

easy

A. Multiplies 5 and 3 to get 15

B. Adds 5 and 3 to get 8

C. Divides 5 by 3 to get 1.67

D. Subtracts 3 from 5 to get 2

Solution

1. Step 1: Understand the $add operator

   The $add operator sums the numbers inside the array.

2. Step 2: Calculate the sum of 5 and 3

   5 + 3 equals 8.

3. Final Answer:

   Adds 5 and 3 to get 8 -> Option B

4. Quick Check:

   $add: [5, 3] = 8 [OK]

Hint: Remember $add sums all values inside the array [OK]

Common Mistakes:

• Confusing $add with $multiply
• Thinking $add subtracts values
• Assuming $add divides numbers

2. Which of the following is the correct syntax to multiply 4 by 7 using MongoDB arithmetic expressions?

easy

A. { $multiply: [4, 7] }

B. { $multiply: 4, 7 }

C. { $multiply: (4, 7) }

D. { multiply: [4, 7] }

Solution

1. Step 1: Recall correct operator syntax

   MongoDB arithmetic operators use $ followed by the operator name and an array of values.

2. Step 2: Check each option

   { $multiply: [4, 7] } uses $multiply with an array [4, 7], which is correct syntax.

3. Final Answer:

   { $multiply: [4, 7] } -> Option A

4. Quick Check:

   Correct syntax uses $multiply with array [OK]

Hint: Use square brackets [] for values in $multiply [OK]

Common Mistakes:

• Omitting $ before multiply
• Using parentheses instead of brackets
• Passing values without array brackets

3. Given the MongoDB aggregation expression { $divide: [ { $multiply: [10, 5] }, 2 ] }, what is the result?

medium

A. 25

B. 100

C. 2.5

D. 50

Solution

1. Step 1: Calculate the inner $multiply expression

   $multiply: [10, 5] equals 50.

2. Step 2: Divide the result by 2

   $divide: [50, 2] equals 25.

3. Final Answer:

   25 -> Option A

4. Quick Check:

   (10 * 5) / 2 = 25 [OK]

Hint: Calculate inner expressions first, then outer [OK]

Common Mistakes:

• Dividing 10 by 5 instead of multiplying
• Forgetting to divide after multiplication
• Mixing order of operations

4. Identify the error in this MongoDB expression: { $divide: [10, { $add: 2, 3 }] }

medium

A. There is no error; the expression is correct

B. The $divide operator cannot divide numbers

C. The numbers 10, 2, and 3 must be strings

D. The $add operator must have an array of values, not separate arguments

Solution

1. Step 1: Check $add operator usage

   $add requires an array of values, but { $add: 2, 3 } uses separate arguments incorrectly.

2. Step 2: Confirm correct syntax

   The correct syntax is { $add: [2, 3] } with square brackets.

3. Final Answer:

   The $add operator must have an array of values, not separate arguments -> Option D

4. Quick Check:

   $add needs array syntax [OK]

Hint: Always use square brackets [] for $add values [OK]

Common Mistakes:

• Using commas instead of array brackets
• Assuming $divide cannot divide numbers
• Passing numbers as strings

5. You want to calculate the average price of items stored in a MongoDB collection using aggregation. Which expression correctly calculates the average price if you have total price as { $add: ["$price1", "$price2", "$price3"] } and want to divide by 3?

hard

A. { $multiply: [ { $add: ["$price1", "$price2", "$price3"] }, 3 ] }

B. { $add: [ { $divide: ["$price1", 3] }, { $divide: ["$price2", 3] }, { $divide: ["$price3", 3] } ] }

C. { $divide: [ { $add: ["$price1", "$price2", "$price3"] }, 3 ] }

D. { $divide: [ 3, { $add: ["$price1", "$price2", "$price3"] } ] }

Solution

1. Step 1: Understand the average formula

   Average = (sum of values) / number of values.

2. Step 2: Check each option

   { $divide: [ { $add: ["$price1", "$price2", "$price3"] }, 3 ] } sums prices then divides by 3, matching the average formula.

3. Final Answer:

   { $divide: [ { $add: ["$price1", "$price2", "$price3"] }, 3 ] } -> Option C

4. Quick Check:

   Sum prices then divide by count = average [OK]

Hint: Sum first, then divide by count for average [OK]

Common Mistakes:

• Dividing each price separately then adding
• Multiplying sum by count instead of dividing
• Dividing count by sum instead of sum by count