Conditional expressions ($cond, $switch) in MongoDB

Introduction

Conditional expressions let you choose values based on conditions, like making decisions in real life.

You want to assign a label based on a score, like 'pass' or 'fail'.

You need to set a discount rate depending on customer type.

You want to categorize ages into groups like 'child', 'adult', or 'senior'.

You want to replace missing values with a default.

You want to perform different calculations based on a field's value.

Syntax

MongoDB

1. $cond: { if: <condition>, then: <true-case>, else: <false-case> }

2. $switch: {
   branches: [
     { case: <condition1>, then: <result1> },
     { case: <condition2>, then: <result2> },
     ...
   ],
   default: <default-result>
}

$cond works like an if-else: if condition is true, return then, else return else.

$switch checks multiple conditions in order and returns the first matching then value, or default if none match.

Examples

Returns 'pass' if score is 60 or more, otherwise 'fail'.

MongoDB

{ $cond: { if: { $gte: ["$score", 60] }, then: "pass", else: "fail" } }

Classifies age into child, teen, adult, or senior.

MongoDB

{ $switch: {
  branches: [
    { case: { $lt: ["$age", 13] }, then: "child" },
    { case: { $lt: ["$age", 20] }, then: "teen" },
    { case: { $lt: ["$age", 65] }, then: "adult" }
  ],
  default: "senior"
} }

Sample Program

This query projects each student's name and score, then adds a 'result' field showing 'Passed' or 'Failed' based on score. It also adds a 'category' field classifying the score into 'Excellent', 'Good', 'Average', or 'Poor'.

MongoDB

db.students.aggregate([
  {
    $project: {
      name: 1,
      score: 1,
      result: {
        $cond: {
          if: { $gte: ["$score", 50] },
          then: "Passed",
          else: "Failed"
        }
      },
      category: {
        $switch: {
          branches: [
            { case: { $gte: ["$score", 90] }, then: "Excellent" },
            { case: { $gte: ["$score", 75] }, then: "Good" },
            { case: { $gte: ["$score", 50] }, then: "Average" }
          ],
          default: "Poor"
        }
      }
    }
  }
])

Important Notes

Use $cond for simple if-else decisions.

Use $switch when you have many conditions to check in order.

Conditions inside $cond and $switch use MongoDB query expressions like $gte, $lt, etc.

Summary

$cond and $switch help you pick values based on conditions.

$cond is like a simple if-else; $switch handles multiple cases.

They make your queries smarter by adding logic inside aggregation.

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