Conditional expressions ($cond, $switch) in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $cond operator do in MongoDB?

The $cond operator works like an if-then-else statement. It checks a condition and returns one value if true, and another if false.

intermediate

How is the $switch operator different from $cond?

$switch lets you check multiple conditions in order, like a switch-case statement. It returns the value for the first true condition.

beginner

Write a simple $cond expression to check if a field score is greater than 50 and return 'Pass' or 'Fail'.

{ $cond: { if: { $gt: ["$score", 50] }, then: "Pass", else: "Fail" } }

intermediate

What happens if none of the conditions in $switch match and no default is provided?

If no conditions match and no default is set, $switch returns null.

beginner

Why are conditional expressions useful in MongoDB aggregation?

They let you create new fields or change values based on conditions, making your data queries smarter and more flexible.

Which operator in MongoDB acts like an if-then-else statement?

A $match

B $switch

C $cond

D $group

$cond checks a condition and returns one value if true, another if false.

What does $switch do when multiple conditions are true?

A Returns the value of the last true condition

B Returns the value of the first true condition

C Returns all matching values

D Returns null

$switch returns the value for the first condition that is true.

In a $cond expression, what does the else part represent?

A Value if condition is false

B The field name

C The condition to check

D Value if condition is true

else is the value returned when the condition is false.

If no conditions match in $switch and no default is set, what is returned?

A null

B The first condition's value

C An error

D An empty string

Without a matching case or default, $switch returns null.

Which of these is a valid use of $cond?

A To filter documents in a collection

B To sort documents

C To group documents by a field

D To assign a value based on a condition

$cond assigns values conditionally inside aggregation pipelines.

Explain how the $cond operator works in MongoDB and give a simple example.

Think of it like an if-else statement in programming.

Describe the purpose of the $switch operator and how it handles multiple conditions.

It is similar to switch-case statements in programming languages.