$mul operator for multiplication in MongoDB - Step-by-Step Execution

Concept Flow - $mul operator for multiplication

Start Update

↓

Find Document

↓

Apply $mul

↓

Multiply Field by Value

↓

Save Updated Document

↓

End Update

The $mul operator multiplies a field's value by a specified number during an update operation.

Execution Sample

MongoDB

db.products.updateOne(
  { _id: 1 },
  { $mul: { price: 1.1 } }
)

This multiplies the 'price' field of the product with _id 1 by 1.1 (increasing price by 10%).

Execution Table

Step	Action	Field 'price' Value Before	Multiplier	Field 'price' Value After	Notes
1	Find document with _id:1	100	N/A	100	Initial price is 100
2	Apply $mul operator	100	1.1	110	100 * 1.1 = 110
3	Save updated document	N/A	N/A	110	Document updated with new price
4	End update	110	N/A	110	Update operation complete

💡 Update ends after multiplying 'price' by 1.1 and saving the document.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
price	100	100	110	110	110

Key Moments - 2 Insights

Why does the 'price' field change from 100 to 110?

Because the $mul operator multiplies the current 'price' value (100) by 1.1, resulting in 110 as shown in execution_table step 2.

What happens if the field does not exist in the document?

The $mul operator will create the field and set it to 0 multiplied by the value, which results in 0. This is not shown in the current example but is important to know.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'price' after step 2?

A 110

B 100

C 1.1

D 0

💡 Hint

Check the 'Field price Value After' column in step 2 of the execution_table.

At which step is the document saved with the updated price?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look for the action 'Save updated document' in the execution_table.

If the multiplier was 2 instead of 1.1, what would be the 'price' after step 2?

A 110

B 200

C 100

D 50

💡 Hint

Multiply the initial price 100 by 2 as per the $mul operator logic.

Concept Snapshot

$mul operator syntax:
{ $mul: { <field>: <number> } }
Multiplies the field's current value by the number.
If field missing, treated as 0.
Used in update operations to scale numeric fields.

Full Transcript

The $mul operator in MongoDB updates multiplies a numeric field by a specified value. The process starts by finding the document to update. Then, $mul multiplies the current field value by the given multiplier. The updated value replaces the old one, and the document is saved. For example, multiplying a price of 100 by 1.1 results in 110. If the field does not exist, it is treated as zero. This operator is useful for scaling prices, quantities, or other numeric data in documents.