Excluding fields from results in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Excluding a single field from MongoDB query results

Given a collection users with documents containing name, age, and email, which query excludes the email field from the results?

MongoDB

db.users.find({}, {email: 0})

A db.users.find({}, {email: 0})

B db.users.find({}, {email: 1})

C db.users.find({}, {email: false})

D db.users.find({}, {email: null})

💡 Hint

Use 0 to exclude a field in the projection document.

In MongoDB, setting a field to 0 in the projection excludes it from the results. Setting it to 1 includes it. Using false or null is invalid syntax.

❓ query_result

intermediate

Excluding multiple fields in MongoDB query results

Which query excludes both password and ssn fields from the accounts collection results?

MongoDB

db.accounts.find({}, {password: 0, ssn: 0})

A db.accounts.find({}, {password: null, ssn: null})

B db.accounts.find({}, {password: 1, ssn: 1})

C db.accounts.find({}, {password: false, ssn: false})

D db.accounts.find({}, {password: 0, ssn: 0})

💡 Hint

Exclude multiple fields by setting each to 0 in the projection.

Setting multiple fields to 0 in the projection excludes them all from the results. Including them with 1 would only show those fields.

📝 Syntax

advanced

Identify the invalid MongoDB projection syntax

Which of the following MongoDB queries will cause a syntax error due to incorrect projection?

A db.products.find({}, {price: false, name: 0})

B db.products.find({}, {price: 1, name: 0})

C db.products.find({}, {price: 0, name: 1})

D db.products.find({}, {price: 0, name: 0})

💡 Hint

Projection values must be 0 or 1, not false.

MongoDB projection only accepts 0 or 1 to exclude or include fields. Using false causes a syntax error.

🧠 Conceptual

advanced

Understanding exclusion and inclusion in MongoDB projections

What happens if you try to exclude one field and include another in the same MongoDB projection, like {field1: 0, field2: 1}?

A MongoDB excludes both fields regardless of the values.

B MongoDB excludes field1 and includes field2 without error.

C MongoDB throws an error because you cannot mix inclusion and exclusion except for _id.

D MongoDB ignores the exclusion and includes all fields.

💡 Hint

MongoDB projection rules restrict mixing inclusion and exclusion.

MongoDB does not allow mixing inclusion and exclusion in the same projection except for the _id field. Doing so causes an error.

❓ optimization

expert

Optimizing MongoDB queries by excluding large fields

You have a collection videos where each document has a large videoData field. To improve query speed when listing videos, which projection is best?

A db.videos.find({}, {videoData: 1})

B db.videos.find({}, {videoData: 0})

C db.videos.find({}, {videoData: null})

D db.videos.find({}, {videoData: false})

💡 Hint

Exclude large fields to reduce data transfer and improve speed.

Excluding the large videoData field with videoData: 0 reduces the amount of data sent, improving query speed. Including it or using invalid values does not help.

Practice

1. What does setting a field to 0 in a MongoDB query projection do?

easy

A. It excludes that field from the query results.

B. It includes that field in the query results.

C. It renames the field in the query results.

D. It sorts the results by that field.

Solution

1. Step 1: Understand projection in MongoDB

   Projection controls which fields appear in the query results.

2. Step 2: Interpret setting a field to 0

   Setting a field to 0 means exclude that field from the results.

3. Final Answer:

   It excludes that field from the query results. -> Option A

4. Quick Check:

   Field set to 0 = excluded [OK]

Hint: Use 0 in projection to exclude fields quickly [OK]

Common Mistakes:

• Thinking 0 includes the field
• Confusing exclusion with sorting
• Mixing exclusion and inclusion in projection

2. Which of the following is the correct syntax to exclude the field password from the results in a MongoDB query?

easy

A. db.users.find({}, {password: 0})

B. db.users.find({}, {password: 1})

C. db.users.find({password: 0})

D. db.users.find({}, {exclude: 'password'})

Solution

1. Step 1: Identify correct projection syntax

   Projection is the second argument to find(), with field names and 0 or 1 values.

2. Step 2: Exclude field by setting it to 0

   Setting password: 0 excludes it from results.

3. Final Answer:

   db.users.find({}, {password: 0}) -> Option A

4. Quick Check:

   Exclude field = field: 0 in projection [OK]

Hint: Use {field: 0} in second find() argument to exclude [OK]

Common Mistakes:

• Using 1 instead of 0 to exclude
• Putting projection inside query filter
• Using non-existent 'exclude' keyword

3. Given the collection products with documents like {_id: 1, name: 'Pen', price: 5, stock: 100}, what will be the result of db.products.find({}, {price: 0, stock: 0})?

medium

A. [{name: 'Pen', price: 5, stock: 100}]

B. [{name: 'Pen'}]

C. [{_id: 1, price: 5, stock: 100}]

D. [{_id: 1, name: 'Pen'}]

Solution

1. Step 1: Understand projection excludes price and stock

   Setting price: 0 and stock: 0 excludes these fields from results.

2. Step 2: Check which fields remain

   _id is included by default, name is included as not excluded.

3. Final Answer:

   [{_id: 1, name: 'Pen'}] -> Option D

4. Quick Check:

   Excluded fields missing, others present [OK]

Hint: Excluded fields disappear; others stay including _id by default [OK]

Common Mistakes:

• Expecting excluded fields to appear
• Forgetting _id is included by default
• Assuming all fields are excluded

4. What is wrong with this MongoDB query to exclude email and include name fields: db.users.find({}, {email: 0, name: 1})?

medium

A. You should use exclude keyword instead of 0.

B. The syntax is correct and will work as expected.

C. You cannot mix exclusion and inclusion except for _id.

D. You must exclude _id explicitly when mixing fields.

Solution

1. Step 1: Recall projection rules

   MongoDB does not allow mixing inclusion and exclusion in projection except for _id.

2. Step 2: Analyze the query

   Query mixes email: 0 (exclude) and name: 1 (include), which is invalid.

3. Final Answer:

   You cannot mix exclusion and inclusion except for _id. -> Option C

4. Quick Check:

   Mixing 0 and 1 fields (except _id) = error [OK]

Hint: Exclude or include fields, but don't mix except _id [OK]

Common Mistakes:

• Mixing 0 and 1 in projection
• Assuming syntax is always flexible
• Using 'exclude' keyword which doesn't exist

5. You want to query orders collection and exclude creditCardNumber and cvv fields for security, but include all other fields including _id. Which query achieves this correctly?

hard

A. db.orders.find({}, {creditCardNumber: 1, cvv: 1})

B. db.orders.find({}, {creditCardNumber: 0, cvv: 0})

C. db.orders.find({}, {creditCardNumber: 0, cvv: 0, _id: 0})

D. db.orders.find({}, {creditCardNumber: false, cvv: false})

Solution

1. Step 1: Exclude sensitive fields by setting them to 0

   Setting creditCardNumber and cvv to 0 excludes them from results.

2. Step 2: Keep _id included by default

   Not excluding _id means it stays included, as required.

3. Final Answer:

   db.orders.find({}, {creditCardNumber: 0, cvv: 0}) -> Option B

4. Quick Check:

   Exclude sensitive fields only, keep others including _id [OK]

Hint: Exclude sensitive fields with 0, leave others alone [OK]

Common Mistakes:

• Excluding _id unintentionally
• Using 1 to exclude fields
• Using false instead of 0