Conditional expressions ($cond, $switch) in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of $cond in aggregation

Given the following MongoDB aggregation pipeline stage, what will be the value of the field status in the output document?

{ $project: { score: 85, status: { $cond: { if: { $gte: ["$score", 60] }, then: "Pass", else: "Fail" } } } }

A {"score": 85, "status": false}

B {"score": 85, "status": "Fail"}

C {"score": 85, "status": true}

D {"score": 85, "status": "Pass"}

💡 Hint

Think about whether the score is greater than or equal to 60.

The $cond expression checks if score is >= 60. Since 85 >= 60 is true, it returns "Pass".

❓ query_result

intermediate

Using $switch for multiple conditions

What will be the value of the field grade after applying this aggregation stage?

{ $project: { score: 75, grade: { $switch: { branches: [ { case: { $gte: ["$score", 90] }, then: "A" }, { case: { $gte: ["$score", 80] }, then: "B" }, { case: { $gte: ["$score", 70] }, then: "C" } ], default: "F" } } } }

A {"score": 75, "grade": "A"}

B {"score": 75, "grade": "B"}

C {"score": 75, "grade": "C"}

D {"score": 75, "grade": "F"}

💡 Hint

Check the conditions in order from top to bottom.

The $switch checks each case in order. 75 is not >= 90 or 80, but it is >= 70, so grade is "C".

📝 Syntax

advanced

Identify the syntax error in $cond usage

Which option contains a syntax error in the use of the $cond expression in MongoDB aggregation?

A { $project: { result: { $cond: { condition: { $eq: ["$value", 10] }, then: "Ten", else: "Other" } } } }

B { $project: { result: { $cond: { if: { $eq: ["$value", 10] }, then: "Ten", else: "Other" } } } }

C { $project: { result: { $cond: [ { $eq: ["$value", 10] }, "Ten", "Other" ] } } }

D } } } } "rehtO" :esle ,"neT" :neht ,} ]01 ,"eulav$"[ :qe$ { :fi { :dnoc$ { :tluser { :tcejorp$ {

💡 Hint

Check the correct field name for the condition in $cond.

The $cond operator requires the field 'if' for the condition, not 'condition'. Option A uses 'condition' which is invalid syntax.

❓ optimization

advanced

Optimizing $switch with overlapping conditions

Consider this $switch expression:

{ $switch: { branches: [ { case: { $gte: ["$score", 70] }, then: "Pass" }, { case: { $gte: ["$score", 90] }, then: "Excellent" } ], default: "Fail" } }

What is the problem with this $switch and how to fix it?

A The $switch should use $cond instead for better performance.

B The order of branches is wrong; the condition for 90 should come before 70 to avoid always matching 70 first.

C The default value should be "Pass" instead of "Fail".

D There is no problem; the $switch works as expected.

💡 Hint

Think about how $switch evaluates branches in order.

Since $switch checks branches in order, the first condition ($gte: 70) matches all scores >= 70, including those >= 90, so the second branch is never reached. Reordering fixes this.

🧠 Conceptual

expert

Behavior of $cond with missing else branch

What happens if you use $cond with only if and then fields, but omit the else field in MongoDB aggregation?

A If the condition is false, the output field will be null.

B If the condition is false, the output field will be missing from the document.

C MongoDB throws a syntax error because else is required.

D If the condition is false, the output field will have the value false.

💡 Hint

Think about default behavior when else is not specified.

When else is omitted, $cond returns null if the condition is false.