createIndex method in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the createIndex method do in MongoDB?

It creates an index on a collection to improve the speed of queries by organizing data for faster search.

beginner

How do you create a simple ascending index on the field name using createIndex?

Use db.collection.createIndex({ name: 1 }). The 1 means ascending order.

intermediate

What is the difference between ascending (1) and descending (-1) in createIndex?

Ascending (1) sorts the index from smallest to largest, descending (-1) sorts from largest to smallest. Both speed up queries but order matters for sorting.

intermediate

What option would you use with createIndex to ensure no duplicate values in the indexed field?

Use the option { unique: true } to prevent duplicate values in the indexed field.

intermediate

Can createIndex be used to create compound indexes? If yes, how?

Yes. You pass multiple fields in the object, like { field1: 1, field2: -1 } to create a compound index on both fields.

What is the main purpose of the createIndex method in MongoDB?

A To create a new collection

B To speed up query performance

C To backup the database

D To delete documents

Indexes help MongoDB find data faster, improving query speed.

Which of the following is the correct syntax to create an ascending index on the field age?

A db.collection.createIndex('age')

B db.collection.createIndex({ age: 'asc' })

C db.collection.createIndex({ age: 1 })

D db.collection.createIndex({ age: true })

The correct syntax uses 1 for ascending order.

How do you create a unique index on the field email?

A db.collection.createIndex({ email: 1 }, { unique: true })

B db.collection.createIndex({ email: 1 }, { unique: false })

C db.collection.createIndex({ email: 1 }, { duplicate: false })

D db.collection.createIndex({ email: 1 })

The option { unique: true } enforces uniqueness.

What does a compound index do?

A Indexes only one field

B Creates a backup of indexes

C Deletes multiple indexes

D Indexes multiple fields together

Compound indexes speed up queries involving multiple fields.

What does the value -1 mean in createIndex?

A Descending order

B Ascending order

C No order

D Unique index

-1 specifies descending order for the index.

Explain how to create a unique ascending index on the field 'username' in MongoDB.

Think about the syntax: db.collection.createIndex({ field: order }, { unique: true })

Describe the benefits of using the createIndex method in MongoDB.

Focus on performance and data integrity.

Practice

1. What is the main purpose of the createIndex method in MongoDB?

easy

A. To speed up queries by creating an index on a field

B. To delete documents from a collection

C. To update multiple documents at once

D. To create a new collection

Solution

1. Step 1: Understand what createIndex does

   The createIndex method organizes data to make searches faster.

2. Step 2: Identify the correct purpose

   It does not delete or update documents, nor create collections, only speeds up queries.

3. Final Answer:

   To speed up queries by creating an index on a field -> Option A

4. Quick Check:

   createIndex speeds up queries [OK]

Hint: Remember: createIndex speeds up searches, not data changes [OK]

Common Mistakes:

• Confusing createIndex with delete or update operations
• Thinking createIndex creates collections
• Assuming createIndex modifies documents

2. Which of the following is the correct syntax to create an ascending index on the field age in MongoDB?

easy

A. db.collection.createIndex({ age: 1 })

B. db.collection.createIndex(age: 1)

C. db.collection.createIndex({ age: -1 })

D. db.collection.createIndex({ age })

Solution

1. Step 1: Recall syntax for createIndex

   The method requires an object with field names as keys and 1 or -1 as values for ascending or descending.

2. Step 2: Identify correct ascending index syntax

   { age: 1 } means ascending index on age. Other options are either descending (-1) or invalid syntax.

3. Final Answer:

   db.collection.createIndex({ age: 1 }) -> Option A

4. Quick Check:

   Ascending index uses 1 [OK]

Hint: Use 1 for ascending, -1 for descending in createIndex [OK]

Common Mistakes:

• Using -1 for ascending index
• Missing curly braces around field and order
• Passing field without object braces

3. Given the command db.users.createIndex({ email: 1 }, { unique: true }), what will happen if you try to insert two documents with the same email value?

medium

A. The index will be ignored and duplicates allowed

B. Both documents will be inserted without error

C. The second insert will fail due to duplicate key error

D. MongoDB will automatically update the first document

Solution

1. Step 1: Understand unique index effect

   Unique index prevents duplicate values in the indexed field.

2. Step 2: Predict behavior on duplicate insert

   Inserting a duplicate email will cause an error and reject the second insert.

3. Final Answer:

   The second insert will fail due to duplicate key error -> Option C

4. Quick Check:

   Unique index blocks duplicates [OK]

Hint: Unique index stops duplicate values in that field [OK]

Common Mistakes:

• Assuming duplicates are allowed with unique index
• Thinking MongoDB auto-updates duplicates
• Believing index is ignored on duplicates

4. What is wrong with this command? db.products.createIndex({ price: 'ascending' })

medium

A. The field name must be a string

B. The collection name is missing

C. createIndex cannot be used on numeric fields

D. The index order must be 1 or -1, not 'ascending'

Solution

1. Step 1: Check index order values

   MongoDB requires 1 for ascending or -1 for descending, not strings like 'ascending'.

2. Step 2: Identify error cause

   Using 'ascending' causes syntax error because it's not a valid order value.

3. Final Answer:

   The index order must be 1 or -1, not 'ascending' -> Option D

4. Quick Check:

   Index order must be 1 or -1 [OK]

Hint: Use 1 or -1 for index order, not words [OK]

Common Mistakes:

• Using string values instead of 1 or -1
• Thinking createIndex can't index numbers
• Confusing collection name with index fields

5. You want to create a compound index on lastName (ascending) and age (descending) fields, ensuring no two documents have the same combination. Which command is correct?

hard

A. db.collection.createIndex({ lastName: -1, age: 1 }, { unique: false })

B. db.collection.createIndex({ lastName: 1, age: -1 }, { unique: true })

C. db.collection.createIndex({ lastName: 1, age: 1 }, { unique: true })

D. db.collection.createIndex({ lastName: 'asc', age: 'desc' }, { unique: true })

Solution

1. Step 1: Understand compound index syntax

   Compound index uses an object with multiple fields and their order: 1 for ascending, -1 for descending.

2. Step 2: Check uniqueness and order

   db.collection.createIndex({ lastName: 1, age: -1 }, { unique: true }) matches ascending lastName (1), descending age (-1), and unique true to prevent duplicates.

3. Step 3: Verify other options

   db.collection.createIndex({ lastName: -1, age: 1 }, { unique: false }) reverses order and is not unique; C uses ascending age instead of descending; D uses invalid string values.

4. Final Answer:

   db.collection.createIndex({ lastName: 1, age: -1 }, { unique: true }) -> Option B

5. Quick Check:

   Compound unique index uses correct order and unique:true [OK]

Hint: Use 1/-1 for order and unique:true for no duplicates [OK]

Common Mistakes:

• Using strings 'asc'/'desc' instead of 1/-1
• Setting unique:false when uniqueness is needed
• Mixing ascending and descending order incorrectly