$inc operator for incrementing in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $inc operator do in MongoDB?

The $inc operator increases or decreases the value of a field by a specified amount. It is used to update numeric values without replacing the whole document.

beginner

How do you increment a field named score by 5 using $inc?

Use { $inc: { score: 5 } } in the update query to add 5 to the current value of score.

beginner

Can $inc be used to decrement a value? How?

Yes, by using a negative number. For example, { $inc: { score: -3 } } decreases the score by 3.

intermediate

What happens if the field you want to increment does not exist?

MongoDB creates the field and sets it to the increment value. For example, if score doesn't exist, { $inc: { score: 1 } } creates score with value 1.

intermediate

Is $inc atomic in MongoDB? Why is this important?

Yes, $inc is atomic, meaning it safely increments the value even if multiple updates happen at the same time. This prevents conflicts and data loss.

What does { $inc: { count: 2 } } do in MongoDB?

A Adds 2 to the current value of <code>count</code>

B Sets <code>count</code> to 2

C Deletes the <code>count</code> field

D Multiplies <code>count</code> by 2

$inc adds the specified number to the current value of the field.

If score does not exist, what will { $inc: { score: 1 } } do?

A Set <code>score</code> to null

B Throw an error

C Ignore the update

D Create <code>score</code> with value 1

MongoDB creates the field and sets it to the increment value if it does not exist.

How do you decrease a field value by 4 using $inc?

A { $dec: { field: 4 } }

B { $inc: { field: 4 } }

C { $inc: { field: -4 } }

D { $dec: { field: -4 } }

Using a negative number with $inc decreases the field value.

Is $inc operation atomic in MongoDB?

A No, it can cause conflicts

B Yes, it prevents race conditions

C Only on replica sets

D Only on sharded clusters

$inc is atomic, so it safely updates values even with concurrent operations.

Which of the following is a valid use of $inc?

A { $inc: { age: 1 } }

B { $inc: { status: true } }

C { $inc: { active: 'yes' } }

D { $inc: { name: 1 } }

$inc works only on numeric fields like age.

Explain how the $inc operator works in MongoDB and give an example of incrementing and decrementing a field.

Think about adding or subtracting numbers from a field.

Why is atomicity important for the $inc operator in MongoDB?

Consider what happens if two people try to update the same number at once.