Boolean and null types in MongoDB

Introduction

Boolean and null types help store simple true/false values and empty or missing information in your data.

To mark if a user is active or inactive with true or false.

To show if a task is completed or not using true or false.

To represent missing or unknown data with null.

To check if a field has no value in a document.

To filter documents based on true/false or null values.

Syntax

MongoDB

{ "fieldName": true | false | null }

Boolean values are either true or false without quotes.

Null means no value and is written as null without quotes.

Examples

This document field shows the user is active.

MongoDB

{ "isActive": true }

This field shows the task is not completed.

MongoDB

{ "isCompleted": false }

This field means the middle name is missing or unknown.

MongoDB

{ "middleName": null }

Sample Program

This example adds three tasks with different boolean and null values for completion. Then it finds only the tasks marked as completed (true).

MongoDB

db.tasks.insertMany([
  { "task": "Wash dishes", "isCompleted": false },
  { "task": "Do homework", "isCompleted": true },
  { "task": "Read book", "isCompleted": null }
])

// Find tasks that are completed
 db.tasks.find({ "isCompleted": true })

Important Notes

Boolean values are useful for yes/no or on/off states.

Null is different from false or empty string; it means no value at all.

When querying, use true, false, or null without quotes to match these types.

Summary

Boolean stores true or false values.

Null represents missing or unknown data.

Use these types to keep data clear and easy to check.

Practice

1. What does the Boolean type represent in MongoDB?

easy

A. Text strings

B. Numbers only

C. True or false values

D. Dates and times

Solution

1. Step 1: Understand Boolean type meaning

   Boolean type stores only two possible values: true or false.

2. Step 2: Compare with other data types

   Numbers, strings, and dates are different types, not Boolean.

3. Final Answer:

   True or false values -> Option C

4. Quick Check:

   Boolean = true/false [OK]

Hint: Boolean means true or false only [OK]

Common Mistakes:

• Confusing Boolean with numbers
• Thinking Boolean stores text
• Mixing Boolean with date types

2. Which of the following is the correct way to store a null value in a MongoDB document?

easy

A. { "field": "null" }

B. { "field": false }

C. { "field": 0 }

D. { "field": null }

Solution

1. Step 1: Identify null value syntax

   In MongoDB, null is stored as the keyword null without quotes.

2. Step 2: Check other options

   "null" is a string, 0 is a number, false is Boolean, so they are incorrect.

3. Final Answer:

   { "field": null } -> Option D

4. Quick Check:

   Null stored as null keyword [OK]

Hint: Use null without quotes for null values [OK]

Common Mistakes:

• Using "null" as a string instead of null
• Confusing 0 or false with null
• Putting null in quotes

3. Given the MongoDB collection documents:

{ "active": true }, { "active": false }, { "active": null }

What will the query db.collection.find({ "active": { $eq: null } }) return?

medium

A. Documents where active is true

B. Documents where active is null

C. Documents where active is false

D. No documents

Solution

1. Step 1: Understand $eq: null behavior

   In MongoDB, querying with { field: { $eq: null } } matches documents where field exists and is exactly null. It does not match missing fields.

2. Step 2: Apply to given documents

   Only the document with "active": null matches; documents with true or false do not.

3. Final Answer:

   Documents where active is null -> Option B

4. Quick Check:

   $eq: null matches existing null [OK]

Hint: $eq: null matches existing null fields [OK]

Common Mistakes:

• Thinking $eq: null matches missing fields
• Confusing false or true as null
• Expecting no results

4. You wrote this MongoDB query to find documents where isActive is false:

db.users.find({ isActive: False })

But it returns an error. What is the problem?

medium

A. Boolean false must be lowercase: false

B. Field name should be in quotes

C. Use $eq operator for Boolean

D. MongoDB does not support Boolean queries

Solution

1. Step 1: Check Boolean syntax in MongoDB queries

   Boolean values in MongoDB queries must be lowercase: true or false.

2. Step 2: Identify error cause

   Using capitalized False causes a syntax error because it's not recognized.

3. Final Answer:

   Boolean false must be lowercase: false -> Option A

4. Quick Check:

   Boolean literals are lowercase in queries [OK]

Hint: Use lowercase true/false in queries [OK]

Common Mistakes:

• Using capitalized True or False
• Omitting quotes on field names (allowed but not error)
• Thinking $eq is required for Boolean

5. You want to find documents where the field verified is either false or null. Which MongoDB query correctly finds these documents?

hard

A. { verified: { $in: [false, null] } }

B. { verified: false }

C. { verified: { $ne: true } }

D. { verified: { $exists: false } }

Solution

1. Step 1: Understand the requirement

   We want documents where verified is false OR null.

2. Step 2: Analyze each option

   { verified: false } matches only false, not null. { verified: { $ne: true } } matches existing fields != true (false, null, other values), but not missing fields and includes unwanted values. { verified: { $exists: false } } matches only missing, not false. { verified: { $in: [false, null] } } uses $in to match false or null exactly.

3. Final Answer:

   { verified: { $in: [false, null] } } -> Option A

4. Quick Check:

   $in matches multiple values including null [OK]

Hint: Use $in with false and null to match both [OK]

Common Mistakes:

• Using only false without null
• Using $exists which misses false
• Using $ne: true which matches unwanted values