Boolean and null types in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What values can a Boolean type hold in MongoDB?

A Boolean type in MongoDB can hold only two values: true or false. These represent simple yes/no or on/off states.

beginner

What does the null type represent in MongoDB?

The null type in MongoDB represents the absence of a value or a missing field. It means the field exists but has no value assigned.

intermediate

How do you query documents where a field is null in MongoDB?

Use { fieldName: null } in your query to find documents where fieldName is either null or does not exist.

beginner

How do you query documents where a Boolean field is true?

Use { fieldName: true } in your query to find documents where fieldName is exactly true.

intermediate

What is the difference between a field set to null and a field that does not exist in MongoDB?

A field set to null exists in the document but has no value. A field that does not exist is completely missing from the document. Queries for { field: null } match both cases.

Which of the following is a valid Boolean value in MongoDB?

A true

B "true"

C 1

D yes

MongoDB Boolean values are strictly true or false without quotes.

What does a null value mean in a MongoDB document?

A The field exists but has no value

B The field is missing

C The field is set to false

D The field is set to zero

null means the field exists but has no assigned value.

How do you find documents where a Boolean field is false?

A { fieldName: 0 }

B { fieldName: null }

C { fieldName: false }

D { fieldName: 'false' }

Use the Boolean value false without quotes to query Boolean false.

Which query matches documents where a field is either null or missing?

A { fieldName: '' }

B { fieldName: false }

C { fieldName: undefined }

D { fieldName: null }

Querying with null matches both null values and missing fields.

If a field is missing in a document, what will a query { fieldName: true } return?

A Documents where fieldName is false

B Documents where fieldName is true

C Documents where fieldName is null

D Documents where fieldName is missing

The query matches only documents where the field exists and is true.

Explain how Boolean and null types are used in MongoDB documents and how to query them.

Think about yes/no values and empty or missing fields.

Describe the difference between a field set to null and a field that does not exist in MongoDB.

Consider presence versus absence of the field.

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