Why result control matters in MongoDB - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find all documents in the 'users' collection.

MongoDB

db.users.[1]()

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

Afind

Binsert

Cupdate

Ddelete

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'insert' instead of 'find' to get data.

Trying to update or delete when just reading data.

The find() method retrieves documents from a collection. Other methods like insert(), update(), and delete() modify data instead.

2fill in blank

medium

Complete the code to limit the number of documents returned to 5.

MongoDB

db.users.find().[1](5)

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

Askip

Bcount

Climit

Dsort

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'skip' to limit results instead of 'limit'.

Using 'count' which returns a number, not documents.

The limit() method restricts the number of documents returned. skip() skips documents, sort() orders them, and count() returns the number of documents.

3fill in blank

hard

Fix the error in the code to sort users by age in ascending order.

MongoDB

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

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

Alimit

Bsort

Cskip

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'limit' or 'skip' to order results.

Using 'count' which returns a number, not sorted documents.

The sort() method orders documents. Using sort({ age: 1 }) sorts by age ascending. Other methods do not order results.

4fill in blank

hard

Fill both blanks to skip the first 3 documents and limit the results to 4.

MongoDB

db.users.find().[1](3).[2](4)

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

Askip

Blimit

Csort

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the order of skip and limit.

Using 'sort' or 'count' instead of skip or limit.

skip(3) skips the first 3 documents, and limit(4) limits the output to 4 documents. This combination controls which results you see.

5fill in blank

hard

Fill all three blanks to find users older than 25, sort by name ascending, and limit to 3 results.

MongoDB

db.users.find({ age: { [1]: 25 } }).[2]({ name: 1 }).[3](3)

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

A$gt

Bsort

Climit

D$lt

Attempts:

3 left

💡 Hint

Common Mistakes

Using '$lt' instead of '$gt' for filtering.

Mixing up the order of sort and limit.

Forgetting to use a filter operator.

$gt means 'greater than' to filter ages above 25. sort({ name: 1 }) orders by name ascending. limit(3) restricts results to 3 documents.

Practice

1. Why is controlling the result important when querying a MongoDB database?

easy

A. It encrypts the data before sending it.

B. It helps retrieve only the necessary data, improving performance.

C. It duplicates the data for backup purposes.

D. It automatically fixes errors in the database.

Solution

1. Step 1: Understand the purpose of result control

   Result control allows you to specify which data to get, how to sort it, and how many results to return.

2. Step 2: Identify the benefit of retrieving only necessary data

   Getting only needed data reduces load and speeds up queries, improving performance.

3. Final Answer:

   It helps retrieve only the necessary data, improving performance. -> Option B

4. Quick Check:

   Result control = better performance [OK]

Hint: Remember: less data means faster queries [OK]

Common Mistakes:

• Thinking result control fixes database errors
• Confusing result control with data backup
• Assuming result control encrypts data

2. Which of the following is the correct MongoDB syntax to limit query results to 5 documents?

easy

A. db.collection.find().limit(5)

B. db.collection.limit(5).find()

C. db.collection.find(5).limit()

D. db.collection.find().limit = 5

Solution

1. Step 1: Recall the correct method to limit results in MongoDB

   The limit() method is called after find() to restrict the number of documents returned.

2. Step 2: Check each option's syntax

   db.collection.find().limit(5) correctly uses find().limit(5). Other options misuse method order or syntax.

3. Final Answer:

   db.collection.find().limit(5) -> Option A

4. Quick Check:

   Use find().limit(n) to limit results [OK]

Hint: Limit comes after find() in MongoDB queries [OK]

Common Mistakes:

• Placing limit() before find()
• Passing number inside find() instead of limit()
• Assigning limit as a property instead of calling it

3. Given the collection users with documents: {name: 'Anna', age: 30}, {name: 'Ben', age: 25}, {name: 'Cara', age: 35}, what will db.users.find().sort({age: 1}).limit(2) return?

medium

A. [{name: 'Ben', age: 25}, {name: 'Anna', age: 30}]

B. [{name: 'Anna', age: 30}, {name: 'Ben', age: 25}]

C. [{name: 'Cara', age: 35}, {name: 'Anna', age: 30}]

D. [{name: 'Ben', age: 25}, {name: 'Cara', age: 35}]

Solution

1. Step 1: Understand the query operations

   The query sorts users by age ascending (smallest to largest) and limits results to 2 documents.

2. Step 2: Sort and select the first two documents

   Sorted by age ascending: Ben (25), Anna (30), Cara (35). Limiting to 2 returns Ben and Anna.

3. Final Answer:

   [{name: 'Ben', age: 25}, {name: 'Anna', age: 30}] -> Option A

4. Quick Check:

   Sort ascending + limit 2 = Ben, Anna [OK]

Hint: Sort ascending means smallest first, then limit [OK]

Common Mistakes:

• Confusing ascending with descending sort
• Ignoring the limit and returning all documents
• Mixing up document order in the result

4. What is wrong with this MongoDB query to get the top 3 oldest users? db.users.find().limit(3).sort({age: -1})

medium

A. limit() cannot be used with sort().

B. find() must have a filter to use limit().

C. The sort order -1 is invalid in MongoDB.

D. The sort() should come before limit() to work correctly.

Solution

1. Step 1: Check the order of method calls

   In MongoDB, sort() must be called before limit() to sort the full result set before limiting.

2. Step 2: Identify the error in the query

   The query calls limit(3) before sort(), so it limits first, then sorts only those limited documents, giving wrong results.

3. Final Answer:

   The sort() should come before limit() to work correctly. -> Option D

4. Quick Check:

   Sort before limit for correct results [OK]

Hint: Always sort before limiting results [OK]

Common Mistakes:

• Calling limit() before sort()
• Thinking limit() and sort() cannot be combined
• Believing -1 is invalid sort order

5. You want to get the names of the 2 youngest users from a large collection but only need their names, not ages. Which query correctly controls the result to get this?

hard

A. db.users.find().limit(2).sort({age: 1})

B. db.users.find({age: 1}, {name: 1}).limit(2)

C. db.users.find({}, {name: 1, _id: 0}).sort({age: 1}).limit(2)

D. db.users.find({}, {name: 1}).sort({age: -1}).limit(2)

Solution

1. Step 1: Understand the requirements

   We want only the names (exclude _id), sorted by age ascending (youngest first), limited to 2 results.

2. Step 2: Analyze each option

   db.users.find({}, {name: 1, _id: 0}).sort({age: 1}).limit(2) correctly projects only name, excludes _id, sorts by age ascending, and limits to 2. Others have wrong filters, sort order, or method order.

3. Final Answer:

   db.users.find({}, {name: 1, _id: 0}).sort({age: 1}).limit(2) -> Option C

4. Quick Check:

   Project name only + sort ascending + limit 2 [OK]

Hint: Project fields, sort ascending, then limit results [OK]

Common Mistakes:

• Not excluding _id when projecting fields
• Sorting descending instead of ascending
• Calling limit before sort