Implicit AND with multiple conditions in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find documents where age is 25 and city is 'New York'.

MongoDB

db.users.find({ age: 25, city: [1] })

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

A'Los Angeles'

B'New York'

C'Chicago'

D'Miami'

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around the city name.

Using $and operator unnecessarily.

The query uses implicit AND by listing multiple fields inside the same object. Here, city must be 'New York'.

2fill in blank

medium

Complete the code to find documents where status is 'active' and score is greater than 80.

MongoDB

db.players.find({ status: 'active', score: { [1]: 80 } })

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

A$gt

B$lt

C$eq

D$ne

Attempts:

3 left

💡 Hint

Common Mistakes

Using $lt which means less than.

Using $eq inside an object instead of direct value.

The operator $gt means 'greater than'. So score must be greater than 80.

3fill in blank

hard

Fix the error in the query to find documents where type is 'admin' and active is true.

MongoDB

db.accounts.find({ type: [1], active: true })

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

Aadmin

B"admin"

Ctrue

D'admin'

Attempts:

3 left

💡 Hint

Common Mistakes

Not quoting string values causes syntax errors.

Using boolean true as a string.

String values in MongoDB queries must be quoted. So 'admin' is correct.

4fill in blank

hard

Fill both blanks to find documents where price is less than 100 and inStock is true.

MongoDB

db.products.find({ price: { [1]: [2] }, inStock: true })

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

A$lt

B100

Ctrue

D$gt

Attempts:

3 left

💡 Hint

Common Mistakes

Using $gt instead of $lt.

Putting true as a price value.

$lt means less than, so price must be less than 100. inStock is already true.

5fill in blank

hard

Fill all three blanks to find documents where category is 'books', rating is greater than 4, and available is false.

MongoDB

db.items.find({ category: [1], rating: { [2]: [3] }, available: false })

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

A'books'

B$gt

C4

D'bookshelf'

Attempts:

3 left

💡 Hint

Common Mistakes

Not quoting 'books' string.

Using $lt instead of $gt for rating.

Putting rating value as string instead of number.

Category must be 'books' (string quoted), rating greater than 4 ($gt: 4), and available false.

Practice

1. In MongoDB, when you write a query with multiple conditions inside a single find() object, how are these conditions combined by default?

easy

A. They are combined with an implicit OR operator.

B. They cause a syntax error unless $and is used explicitly.

C. They are combined with an implicit AND operator.

D. They are ignored except for the first condition.

Solution

1. Step 1: Understand MongoDB query object behavior

   When multiple conditions are inside one query object, MongoDB treats them as AND conditions automatically.

2. Step 2: Recall the implicit AND rule

   You do not need to write $and explicitly; multiple fields in the query mean all must match.

3. Final Answer:

   They are combined with an implicit AND operator. -> Option C

4. Quick Check:

   Multiple conditions = implicit AND [OK]

Hint: Multiple conditions in one object mean AND automatically [OK]

Common Mistakes:

• Thinking multiple conditions mean OR by default
• Believing $and is always required
• Assuming only the first condition is checked

2. Which of the following is the correct syntax to find documents where age is 25 and status is "active" using implicit AND in MongoDB?

easy

A. db.collection.find({ age: 25 || status: "active" })

B. db.collection.find({ age: 25, status: "active" })

C. db.collection.find({ $and: [ { age: 25 }, { status: "active" } ] })

D. db.collection.find({ age: 25 && status: "active" })

Solution

1. Step 1: Recognize implicit AND syntax

   Using multiple fields inside one object automatically means AND, so { age: 25, status: "active" } is correct.

2. Step 2: Identify incorrect syntax

   Options A and D use JavaScript operators inside query object, which is invalid. db.collection.find({ $and: [ { age: 25 }, { status: "active" } ] }) is correct but explicit $and is not needed here.

3. Final Answer:

   db.collection.find({ age: 25, status: "active" }) -> Option B

4. Quick Check:

   Multiple fields in one object = implicit AND [OK]

Hint: Use multiple fields in one object for AND, no $and needed [OK]

Common Mistakes:

• Using JavaScript operators like && or || inside query object
• Always writing $and even when not needed
• Confusing OR and AND syntax

3. Given the collection documents:

{ name: "Alice", age: 30, city: "NY" }

{ name: "Bob", age: 25, city: "LA" }

{ name: "Carol", age: 30, city: "LA" }

What will be the result of this query?
db.collection.find({ age: 30, city: "LA" })

medium

A. []

B. [{ name: "Alice", age: 30, city: "NY" }, { name: "Carol", age: 30, city: "LA" }]

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

D. [{ name: "Carol", age: 30, city: "LA" }]

Solution

1. Step 1: Understand query conditions

   The query looks for documents where age is 30 AND city is "LA" simultaneously.

2. Step 2: Match documents

   Only Carol has age 30 and city "LA". Alice has age 30 but city "NY". Bob has city "LA" but age 25.

3. Final Answer:

   [{ name: "Carol", age: 30, city: "LA" }] -> Option D

4. Quick Check:

   Implicit AND filters to Carol only [OK]

Hint: All conditions must match; check each field carefully [OK]

Common Mistakes:

• Including documents that match only one condition
• Confusing OR with AND results
• Ignoring city or age condition

4. You wrote this query to find documents where status is "pending" and priority is "high":
db.tasks.find({ status: "pending", $and: [{ priority: "high" }] })
What is wrong with this query?

medium

A. The query will work fine and return correct results.

B. The $and array should contain multiple conditions, not just one.

C. Using $and inside the same object with other fields causes a syntax error.

D. You cannot mix implicit AND and explicit $and in the same query object.

Solution

1. Step 1: Analyze query structure

   The query has an implicit field condition (status: "pending") combined with an explicit $and containing { priority: "high" }.

2. Step 2: Understand MongoDB query rules

   MongoDB allows top-level fields and operators like $and to be combined with implicit AND. $and with a single condition is valid and equivalent to just including the field directly.

3. Final Answer:

   The query will work fine and return correct results. -> Option A

4. Quick Check:

   Mixing implicit AND and $and in one object is valid [OK]

Hint: You can mix implicit AND fields and $and operator at top level [OK]

Common Mistakes:

• Thinking you cannot mix implicit fields and $and in the same object
• Believing $and requires multiple conditions
• Expecting a syntax error with $and containing one condition

5. You want to find all documents in a collection where category is "books", price is less than 20, and inStock is true. Which query uses implicit AND correctly and returns the expected results?

hard

A. db.collection.find({ category: "books", price: { $lt: 20 }, inStock: true })

B. db.collection.find({ $and: [ { category: "books" }, { price: { $lt: 20 } }, { inStock: true } ] })

C. db.collection.find({ category: "books" || price: { $lt: 20 } || inStock: true })

D. db.collection.find({ category: "books", price: { $lt: 20 } && inStock: true })

Solution

1. Step 1: Identify correct implicit AND syntax

   Using multiple fields inside one object combines conditions with implicit AND, so db.collection.find({ category: "books", price: { $lt: 20 }, inStock: true }) is correct and clean.

2. Step 2: Check other options for errors

   db.collection.find({ $and: [ { category: "books" }, { price: { $lt: 20 } }, { inStock: true } ] }) is valid but uses explicit $and unnecessarily. Options A and C use invalid JavaScript operators inside query object, causing errors.

3. Final Answer:

   db.collection.find({ category: "books", price: { $lt: 20 }, inStock: true }) -> Option A

4. Quick Check:

   Multiple fields = implicit AND, no operators needed [OK]

Hint: List all conditions as fields in one object for implicit AND [OK]

Common Mistakes:

• Using JavaScript operators like || or && inside query object
• Overusing $and when implicit AND suffices
• Mixing implicit AND and explicit $and incorrectly