Conditional expressions ($cond, $switch) in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to use $cond to check if the field 'score' is greater than 50.

MongoDB

{ $cond: { if: { $gt: ["$score", [1]] }, then: "Pass", else: "Fail" } }

Drag options to blanks, or click blank then click option'

A50

B100

C0

D75

Attempts:

3 left

💡 Hint

Common Mistakes

Using a value other than 50 for the comparison.

Confusing $gt with $lt.

The $cond expression checks if 'score' is greater than 50. If true, it returns "Pass", else "Fail".

2fill in blank

medium

Complete the $cond expression to return 'High' if 'age' is at least 18, otherwise 'Low'.

MongoDB

{ $cond: { if: { $gte: ["$age", [1]] }, then: "High", else: "Low" } }

Drag options to blanks, or click blank then click option'

A18

B16

C20

D21

Attempts:

3 left

💡 Hint

Common Mistakes

Using 21 instead of 18.

Using $gt instead of $gte.

$gte means 'greater than or equal to'. We check if age is at least 18 to return 'High'.

3fill in blank

hard

Fix the error in the $switch expression to correctly return 'Child', 'Teen', or 'Adult' based on 'age'.

MongoDB

{ $switch: { branches: [ { case: { $lt: ["$age", [1]] }, then: "Child" }, { case: { $lt: ["$age", 20] }, then: "Teen" } ], default: "Adult" } }

Drag options to blanks, or click blank then click option'

A21

B13

C18

D16

Attempts:

3 left

💡 Hint

Common Mistakes

Using 13 or 16 instead of 18 for the first case.

Not ordering cases properly.

The first case should check if age is less than 18 to classify as 'Child'. Then less than 20 is 'Teen'. Otherwise, 'Adult'.

4fill in blank

hard

Fill both blanks to create a $cond expression that returns 'Adult' if age is 18 or more, else 'Minor'.

MongoDB

{ $cond: { if: { $[1]: ["$age", [2]] }, then: "Adult", else: "Minor" } }

Drag options to blanks, or click blank then click option'

Agte

Blt

C18

D21

Attempts:

3 left

💡 Hint

Common Mistakes

Using $lt instead of $gte.

Using 21 instead of 18.

Use $gte to check if age is greater than or equal to 18 to return 'Adult'. Otherwise, return 'Minor'.

5fill in blank

hard

Fill all three blanks to create a $switch expression that returns 'Low', 'Medium', or 'High' based on 'score'.

MongoDB

{ $switch: { branches: [ { case: { $lt: ["$score", [1]] }, then: "Low" }, { case: { $lt: ["$score", [2]] }, then: "Medium" } ], default: [3] } }

Drag options to blanks, or click blank then click option'

A50

B80

C"High"

D"Medium"

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the order of thresholds.

Using incorrect default value.

The first case checks if score is less than 50 for 'Low'. The second checks less than 80 for 'Medium'. Otherwise, 'High'.

Practice

1. What does the $cond operator do in a MongoDB aggregation pipeline?

easy

A. It chooses a value based on a simple if-else condition.

B. It sorts documents by a field value.

C. It groups documents by a key.

D. It removes duplicate documents.

Solution

1. Step 1: Understand the purpose of $cond

   $cond is used to evaluate a condition and return one value if true, another if false, similar to if-else logic.

2. Step 2: Compare with other options

   Sorting, grouping, and removing duplicates are done by other operators like $sort, $group, and $addToSet, not $cond.

3. Final Answer:

   It chooses a value based on a simple if-else condition. -> Option A

4. Quick Check:

   $cond = if-else logic [OK]

Hint: Remember $cond is like a simple if-else choice [OK]

Common Mistakes:

• Confusing $cond with $group or $sort
• Thinking $cond filters documents
• Assuming $cond handles multiple cases like $switch

2. Which of the following is the correct syntax for using $cond in a MongoDB aggregation pipeline?

easy

A. { $cond: [ <condition>, <trueValue>, <falseValue> ] }

B. { $cond: { if: <condition>, then: <trueValue>, else: <falseValue> } }

C. { $cond: { case: <condition>, then: <trueValue>, else: <falseValue> } }

D. { $cond: { when: <condition>, then: <trueValue>, else: <falseValue> } }

Solution

1. Step 1: Recall $cond syntax

   The $cond operator uses an array with three elements: condition, true value, false value.

2. Step 2: Check options

   { $cond: [ <condition>, <trueValue>, <falseValue> ] } matches the correct array syntax. Options A, C, and D use incorrect keys or object format.

3. Final Answer:

   { $cond: [ <condition>, <trueValue>, <falseValue> ] } -> Option A

4. Quick Check:

   $cond syntax = array with 3 elements [OK]

Hint: Use array syntax: condition, true, false for $cond [OK]

Common Mistakes:

• Using object keys like if/then/else instead of array
• Confusing $cond with $switch syntax
• Missing one of the three required elements

3. Given the following aggregation stage, what will be the value of the field status for a document with score: 75?

{
  $addFields: {
    status: {
      $switch: {
        branches: [
          { case: { $gte: ["$score", 90] }, then: "Excellent" },
          { case: { $gte: ["$score", 70] }, then: "Good" },
          { case: { $gte: ["$score", 50] }, then: "Average" }
        ],
        default: "Poor"
      }
    }
  }
}

medium

A. "Poor"

B. "Excellent"

C. "Average"

D. "Good"

Solution

1. Step 1: Evaluate $switch branches in order

   The first branch checks if score >= 90 (75 >= 90? No). The second checks if score >= 70 (75 >= 70? Yes).

2. Step 2: Determine matching branch and output

   Since the second branch matches, the status is set to "Good". Later branches are ignored.

3. Final Answer:

   "Good" -> Option D

4. Quick Check:

   $switch picks first true case = "Good" [OK]

Hint: Check $switch branches top to bottom, pick first true [OK]

Common Mistakes:

• Ignoring order of branches in $switch
• Choosing default instead of matching branch
• Confusing $gte logic

4. Identify the issue in this aggregation expression using $switch:

{
  $project: {
    grade: {
      $switch: {
        branches: [
          { case: { $gt: ["$score", 90] }, then: "A" },
          { case: { $gt: ["$score", 80] }, then: "B" }
        ]
      }
    }
  }
}

medium

A. Branches array must be outside $switch object.

B. Missing default value in $switch returns null for unmatched cases.

C. Using $gt instead of $gte is invalid syntax.

D. The $project stage cannot use $switch.

Solution

1. Step 1: Check $switch structure

   $switch requires branches; default is optional but without it returns null when no branch matches.

2. Step 2: Identify missing default

   The expression lacks a default field, so if score <= 80, grade is null.

3. Final Answer:

   Missing default value in $switch returns null for unmatched cases. -> Option B

4. Quick Check:

   $switch missing default = null [OK]

Hint: Always include default in $switch to avoid errors [OK]

Common Mistakes:

• Omitting default field in $switch
• Thinking $gt is invalid syntax
• Misplacing branches array

5. You want to categorize products by price using $switch in an aggregation pipeline. Prices below 20 are "Cheap", 20 to 50 are "Moderate", above 50 are "Expensive". Which of the following $switch expressions correctly implements this?

hard

A. { $switch: { branches: [ { case: { $gte: ["$price", 50] }, then: "Expensive" }, { case: { $gte: ["$price", 20] }, then: "Moderate" }, { case: { $lt: ["$price", 20] }, then: "Cheap" } ], default: "Unknown" } }

B. { $switch: { branches: [ { case: { $lte: ["$price", 20] }, then: "Cheap" }, { case: { $lt: ["$price", 50] }, then: "Moderate" }, { case: { $gte: ["$price", 50] }, then: "Expensive" } ], default: "Unknown" } }

C. { $switch: { branches: [ { case: { $lt: ["$price", 20] }, then: "Cheap" }, { case: { $and: [ { $gte: ["$price", 20] }, { $lte: ["$price", 50] } ] }, then: "Moderate" }, { case: { $gt: ["$price", 50] }, then: "Expensive" } ], default: "Unknown" } }

D. { $switch: { branches: [ { case: { $gte: ["$price", 50] }, then: "Expensive" }, { case: { $lte: ["$price", 20] }, then: "Cheap" }, { case: { $and: [ { $gt: ["$price", 20] }, { $lt: ["$price", 50] } ] }, then: "Moderate" } ], default: "Unknown" } }

Solution

1. Step 1: Analyze price ranges and conditions

   Prices below 20 are "Cheap", 20 to 50 inclusive are "Moderate", above 50 are "Expensive". Conditions must be mutually exclusive and cover all cases.

2. Step 2: Check each option's logic

   { $switch: { branches: [ { case: { $lt: ["$price", 20] }, then: "Cheap" }, { case: { $and: [ { $gte: ["$price", 20] }, { $lte: ["$price", 50] } ] }, then: "Moderate" }, { case: { $gt: ["$price", 50] }, then: "Expensive" } ], default: "Unknown" } } correctly uses $lt for below 20, $and with $gte and $lte for 20 to 50, and $gt for above 50. Others have overlapping or incorrect ranges.

3. Final Answer:

   { $switch: { branches: [ { case: { $lt: ["$price", 20] }, then: "Cheap" }, { case: { $and: [ { $gte: ["$price", 20] }, { $lte: ["$price", 50] } ] }, then: "Moderate" }, { case: { $gt: ["$price", 50] }, then: "Expensive" } ], default: "Unknown" } } -> Option C

4. Quick Check:

   Use clear, non-overlapping conditions in $switch branches [OK]

Hint: Use clear, non-overlapping conditions with $and for ranges [OK]

Common Mistakes:

• Overlapping ranges causing wrong category
• Using incorrect comparison operators
• Not covering all price values