$or operator behavior in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find documents where the field 'status' is either 'A' or 'D'.

MongoDB

db.collection.find({ $or: [ { status: [1] }, { status: 'D' } ] })

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

A'C'

B'B'

C'A'

D'E'

Attempts:

3 left

💡 Hint

Common Mistakes

Using $or without an array.

Using incorrect quotes or missing quotes around string values.

The $or operator requires an array of conditions. Here, we want documents where status is 'A' or 'D'. So the first condition must be { status: 'A' }.

2fill in blank

medium

Complete the code to find documents where 'age' is less than 30 or 'score' is greater than 80.

MongoDB

db.collection.find({ $or: [ { age: { $lt: [1] } }, { score: { $gt: 80 } } ] })

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

A30

B25

C35

D40

Attempts:

3 left

💡 Hint

Common Mistakes

Using $gt instead of $lt for age.

Using a wrong number for comparison.

The condition is age less than 30, so $lt must be 30.

3fill in blank

hard

Fix the error in the query to correctly find documents where 'type' is 'book' or 'author' is 'John'.

MongoDB

db.collection.find({ $or: [ { type: 'book' }, { author: [1] } ] })

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

AJohn

B[ 'John' ]

C{ 'John' }

D'John'

Attempts:

3 left

💡 Hint

Common Mistakes

Using an object instead of an array for $or.

Not quoting string values.

The $or operator requires an array of conditions, not an object. The value for author should be a string 'John'.

4fill in blank

hard

Fill both blanks to find documents where 'category' is 'fiction' or 'year' is greater than 2010.

MongoDB

db.collection.find({ $or: [ { category: [1] }, { year: { $[2]: 2010 } } ] })

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

A'fiction'

Blt

Cgt

D'non-fiction'

Attempts:

3 left

💡 Hint

Common Mistakes

Using $lt instead of $gt for year.

Not quoting string values for category.

The first condition checks if category equals 'fiction'. The second condition uses $gt (greater than) for year > 2010.

5fill in blank

hard

Fill all three blanks to find documents where 'status' is 'active', or 'score' is at least 90, or 'age' is less than 25.

MongoDB

db.collection.find({ $or: [ { status: [1] }, { score: { $[2]: [3] } }, { age: { $lt: 25 } } ] })

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

A'active'

Bgte

C90

D'inactive'

Attempts:

3 left

💡 Hint

Common Mistakes

Using $gt instead of $gte for score.

Not quoting string values for status.

The first condition checks status 'active'. The second uses $gte (greater than or equal) with 90 for score. The third checks age less than 25.

Practice

1.

What does the $or operator do in MongoDB queries?

easy

A. It returns documents that match at least one of the given conditions.

B. It returns documents that match all given conditions simultaneously.

C. It returns documents that do not match any of the given conditions.

D. It sorts documents based on multiple fields.

Solution

1. Step 1: Understand the purpose of $or

   The $or operator is used to find documents that satisfy at least one condition from multiple conditions.

2. Step 2: Compare with other operators

   Unlike $and, which requires all conditions to be true, $or requires only one condition to be true.

3. Final Answer:

   It returns documents that match at least one of the given conditions. -> Option A

4. Quick Check:

   $or = match any condition [OK]

Hint: Think 'or' means any one condition true [OK]

Common Mistakes:

• Confusing $or with $and operator
• Thinking $or filters documents matching all conditions
• Assuming $or sorts documents

2.

Which of the following is the correct syntax to use $or in a MongoDB query?

{ $or: [ { age: { $lt: 20 } }, { city: "NY" } ] }

easy

A. { $or: { age: { $lt: 20 }, city: "NY" } }

B. { $or: [ age: { $lt: 20 }, city: "NY" ] }

C. { $or: [ { age: { $lt: 20 } }, { city: "NY" } ] }

D. { $or: ( age: { $lt: 20 }, city: "NY" ) }

Solution

1. Step 1: Check the structure of $or

   The $or operator requires an array of condition objects inside square brackets.

2. Step 2: Validate each option's syntax

   { $or: [ { age: { $lt: 20 } }, { city: "NY" } ] } correctly uses an array with two objects. Options A, B, and C use incorrect brackets or missing array syntax.

3. Final Answer:

   { $or: [ { age: { $lt: 20 } }, { city: "NY" } ] } -> Option C

4. Quick Check:

   $or needs array of conditions [OK]

Hint: Remember: $or takes an array of condition objects [OK]

Common Mistakes:

• Using curly braces instead of square brackets for conditions
• Using parentheses instead of brackets
• Not wrapping conditions inside an array

3.

Given the collection users with documents:

[{ "name": "Alice", "age": 25, "city": "NY" }, { "name": "Bob", "age": 19, "city": "LA" }, { "name": "Carol", "age": 30, "city": "SF" }]

What will the query db.users.find({ $or: [ { age: { $lt: 20 } }, { city: "SF" } ] }) return?

medium

A. [{ "name": "Bob", "age": 19, "city": "LA" }, { "name": "Carol", "age": 30, "city": "SF" }]

B. [{ "name": "Alice", "age": 25, "city": "NY" }]

C. [{ "name": "Bob", "age": 19, "city": "LA" }]

D. []

Solution

1. Step 1: Identify documents matching each condition

   Condition 1: age < 20 matches Bob (age 19). Condition 2: city = "SF" matches Carol.

2. Step 2: Combine results with $or

   The query returns documents matching either condition, so Bob and Carol are included.

3. Final Answer:

   [{ "name": "Bob", "age": 19, "city": "LA" }, { "name": "Carol", "age": 30, "city": "SF" }] -> Option A

4. Quick Check:

   $or returns any matching document [OK]

Hint: Check each condition separately, then combine results [OK]

Common Mistakes:

• Including documents that don't match any condition
• Confusing $or with $and and expecting all conditions to match
• Ignoring one of the conditions

4.

Consider this query that causes an error:

db.collection.find({ $or: { age: { $gt: 30 }, city: "NY" } })

What is the main issue causing the error?

medium

A. The field names inside $or must be strings.

B. The query is missing a closing parenthesis.

C. The operator $gt cannot be used inside $or.

D. $or requires an array of conditions, not a single object.

Solution

1. Step 1: Analyze the $or syntax

   $or expects an array of condition objects, but here it is given a single object.

2. Step 2: Identify the error cause

   Because the value is not an array, MongoDB throws a syntax error.

3. Final Answer:

   $or requires an array of conditions, not a single object. -> Option D

4. Quick Check:

   $or needs array syntax [OK]

Hint: Always wrap $or conditions in square brackets [OK]

Common Mistakes:

• Using object instead of array for $or conditions
• Misplacing operators inside $or
• Ignoring syntax errors from missing brackets

5.

You want to find documents in a products collection where the category is either "electronics" or the price is less than 100. Which query correctly uses $or to achieve this?

hard

A. db.products.find({ category: "electronics" && price: { $lt: 100 } })

B. db.products.find({ $or: [ { category: "electronics" }, { price: { $lt: 100 } } ] })

C. db.products.find({ $or: { category: "electronics", price: { $lt: 100 } } })

D. db.products.find({ $and: [ { category: "electronics" }, { price: { $lt: 100 } } ] })

Solution

1. Step 1: Understand the query goal

   We want documents where category is "electronics" OR price is less than 100.

2. Step 2: Check each option's logic and syntax

   db.products.find({ $or: [ { category: "electronics" }, { price: { $lt: 100 } } ] }) correctly uses $or with an array of two conditions. db.products.find({ $or: { category: "electronics", price: { $lt: 100 } } }) uses an object instead of array, causing syntax error. db.products.find({ category: "electronics" && price: { $lt: 100 } }) uses invalid syntax with &&. db.products.find({ $and: [ { category: "electronics" }, { price: { $lt: 100 } } ] }) uses $and, which requires both conditions to be true, not either.

3. Final Answer:

   db.products.find({ $or: [ { category: "electronics" }, { price: { $lt: 100 } } ] }) -> Option B

4. Quick Check:

   $or with array matches either condition [OK]

Hint: Use array inside $or for multiple conditions [OK]

Common Mistakes:

• Using object instead of array for $or
• Confusing $or with $and
• Using invalid logical operators like && in MongoDB queries