$and operator behavior in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find documents where age is greater than 25 and status is "A" using $and.

MongoDB

db.collection.find({ $and: [ { age: { $gt: [1] } }, { status: "A" } ] })

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

A25

B30

C20

D15

Attempts:

3 left

💡 Hint

Common Mistakes

Using a value less than 25 will include unwanted documents.

Using a value greater than 25 will exclude some valid documents.

The $gt operator checks if age is greater than 25, so 25 is the correct value to use.

2fill in blank

medium

Complete the code to find documents where score is at least 80 and grade is "B" using $and.

MongoDB

db.collection.find({ $and: [ { score: { $gte: [1] } }, { grade: "B" } ] })

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

A75

B80

C85

D70

Attempts:

3 left

💡 Hint

Common Mistakes

Using a value less than 80 includes lower scores.

Using a value greater than 80 excludes scores exactly 80.

The $gte operator means 'greater than or equal to'. To find scores at least 80, use 80.

3fill in blank

hard

Fix the error in the query to correctly use $and to find documents where city is "NY" and age is less than 30.

MongoDB

db.collection.find({ $and: [ { city: "NY" }, { age: { $lt: [1] } } ] })

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

Aage

B"30"

C30

D25

Attempts:

3 left

💡 Hint

Common Mistakes

Putting 30 in quotes makes it a string, causing no matches.

Using a field name instead of a value causes syntax errors.

The $lt operator expects a number, so 30 without quotes is correct.

4fill in blank

hard

Fill both blanks to find documents where status is "A" and score is greater than 70 using $and.

MongoDB

db.collection.find({ $and: [ { status: [1] }, { score: { $gt: [2] } } ] })

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

A"A"

B70

C75

D"B"

Attempts:

3 left

💡 Hint

Common Mistakes

Using numbers with quotes or strings without quotes.

Mixing up the values for status and score.

Status is a string, so use "A" with quotes. Score is a number, so use 70 without quotes.

5fill in blank

hard

Fill all three blanks to find documents where age is at least 18, city is "LA", and active is true using $and.

MongoDB

db.collection.find({ $and: [ { age: { $gte: [1] } }, { city: [2] }, { active: [3] } ] })

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

A18

B"LA"

Ctrue

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using strings for numbers or booleans.

Using false instead of true for active.

Age is a number (18), city is a string ("LA"), and active is a boolean (true).

Practice

1.

What does the $and operator do in a MongoDB query?

easy

A. It finds documents that match all the given conditions.

B. It finds documents that match any one of the given conditions.

C. It sorts documents based on multiple fields.

D. It deletes documents that match the conditions.

Solution

1. Step 1: Understand the purpose of $and

   The $and operator combines multiple conditions and requires all to be true for a document to match.

2. Step 2: Compare with other operators

   Unlike $or, which matches if any condition is true, $and needs all conditions true.

3. Final Answer:

   It finds documents that match all the given conditions. -> Option A

4. Quick Check:

   $and means all conditions must match [OK]

Hint: All conditions inside $and must be true to match [OK]

Common Mistakes:

• Confusing $and with $or operator
• Thinking $and sorts documents
• Assuming $and deletes documents

2.

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

{ $and: [ { age: { $gt: 20 } }, { city: "NY" } ] }

easy

A. { $and: [ { age: { $gt: 20 } }, { city: "NY" } ] }

B. { $and: { age: 20, city: "NY" } }

C. { $and: ( { age: { $gt: 20 } }, { city: "NY" } ) }

D. { $and: { age: { $gt: 20 }, city: "NY" } }

Solution

1. Step 1: Recall the syntax of $and

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

2. Step 2: Check each option's structure

   { $and: [ { age: { $gt: 20 } }, { city: "NY" } ] } correctly uses an array with two condition objects. Options A and B use objects instead of arrays, and C uses parentheses which is invalid.

3. Final Answer:

   { $and: [ { age: { $gt: 20 } }, { city: "NY" } ] } -> Option A

4. Quick Check:

   $and needs an array of conditions [OK]

Hint: Use square brackets [] for conditions inside $and [OK]

Common Mistakes:

• Using curly braces {} instead of array []
• Using parentheses () instead of array []
• Putting conditions directly without array

3.

Given the collection users with documents:

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

What will the query { $and: [ { age: 25 }, { city: "LA" } ] } return?

medium

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

B. [{ name: "Bob", age: 30, city: "LA" }]

C. [{ name: "Carol", age: 25, city: "LA" }]

D. []

Solution

1. Step 1: Understand the query conditions

   The query looks for documents where age is 25 AND city is "LA".

2. Step 2: Check each document against conditions

   Alice has age 25 but city "NY" (fails city condition). Bob has city "LA" but age 30 (fails age condition). Carol has age 25 and city "LA" (matches both).

3. Final Answer:

   [{ name: "Carol", age: 25, city: "LA" }] -> Option C

4. Quick Check:

   Both conditions true only for Carol [OK]

Hint: Both conditions must match a document to be returned [OK]

Common Mistakes:

• Selecting documents matching only one condition
• Ignoring the AND logic of $and
• Confusing city names or ages

4.

Consider this query:

{ $and: { age: { $gt: 20 }, city: "NY" } }

What is wrong with this query?

medium

A. The operator $gt cannot be used inside $and.

B. The conditions inside $and must be in an array, not an object.

C. The city value must be a number, not a string.

D. The query is correct and will work as expected.

Solution

1. Step 1: Check the structure of $and

   The $and operator requires an array of conditions, but here it is given an object.

2. Step 2: Identify the error

   Using an object instead of an array causes a syntax error in MongoDB queries.

3. Final Answer:

   The conditions inside $and must be in an array, not an object. -> Option B

4. Quick Check:

   $and needs an array of conditions [OK]

Hint: Always use square brackets [] for $and conditions [OK]

Common Mistakes:

• Using object {} instead of array [] for $and
• Assuming $gt is invalid inside $and
• Thinking string values are not allowed

5.

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

hard

A. { price: { $gt: 100 }, category: { $or: [ "electronics", "appliances" ] } }

B. { $or: [ { price: { $gt: 100 } }, { category: "electronics" }, { category: "appliances" } ] }

C. { $and: { price: { $gt: 100 }, category: { $in: [ "electronics", "appliances" ] } } }

D. { $and: [ { price: { $gt: 100 } }, { $or: [ { category: "electronics" }, { category: "appliances" } ] } ] }

Solution

1. Step 1: Understand the conditions

   The query needs price > 100 AND category is either "electronics" OR "appliances".

2. Step 2: Check each option's logic

   { $and: [ { price: { $gt: 100 } }, { $or: [ { category: "electronics" }, { category: "appliances" } ] } ] } correctly uses $and with price condition and an inner $or for categories. { $or: [ { price: { $gt: 100 } }, { category: "electronics" }, { category: "appliances" } ] } uses $or for all, which is incorrect. { price: { $gt: 100 }, category: { $or: [ "electronics", "appliances" ] } } uses invalid syntax for $or inside category. { $and: { price: { $gt: 100 }, category: { $in: [ "electronics", "appliances" ] } } } uses $and with an object instead of array, which is invalid.

3. Final Answer:

   { $and: [ { price: { $gt: 100 } }, { $or: [ { category: "electronics" }, { category: "appliances" } ] } ] } -> Option D

4. Quick Check:

   Combine $and for price and $or for categories [OK]

Hint: Use $and for all must match, $or inside for alternatives [OK]

Common Mistakes:

• Using $or for all conditions instead of $and
• Incorrect syntax for $or inside category
• Using object instead of array for $and