Composite primary key in DynamoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Querying with Composite Primary Key in DynamoDB

Given a DynamoDB table with a composite primary key consisting of PartitionKey and SortKey, what will be the output of the following query?

Query: Get all items where PartitionKey = 'User#123' and SortKey begins with 'Order#'

DynamoDB

Table: Orders
PartitionKey: User#123
SortKey: Order#001, Order#002, Profile#001

Query: PartitionKey = 'User#123' AND begins_with(SortKey, 'Order#')

A Returns all items with PartitionKey 'User#123' including 'Profile#001'

B Returns items with SortKey 'Order#001' and 'Order#002' only

C Returns no items because begins_with is not valid in DynamoDB

D Returns items with SortKey 'Profile#001' only

💡 Hint

Think about how the sort key condition works with begins_with in DynamoDB queries.

The query filters items by PartitionKey 'User#123' and only includes those where the SortKey starts with 'Order#'. So only 'Order#001' and 'Order#002' are returned.

🧠 Conceptual

intermediate

Understanding Composite Primary Key Components

Which two components make up a composite primary key in DynamoDB?

A Partition Key and Sort Key

B Partition Key and Global Secondary Index

C Hash Key and Range Key

D Primary Key and Secondary Key

💡 Hint

One key decides the partition, the other orders items within that partition.

A composite primary key consists of a Partition Key (also called Hash Key) and a Sort Key (also called Range Key). Together they uniquely identify an item.

📝 Syntax

advanced

Correct Syntax for Querying Composite Primary Key

Which of the following DynamoDB query expressions correctly uses the composite primary key to get all items with PartitionKey 'User#456' and SortKey between 'Order#100' and 'Order#200'?

A KeyConditionExpression: 'PartitionKey = :pk AND SortKey IN (:start, :end)'

B KeyConditionExpression: 'PartitionKey = :pk OR SortKey BETWEEN :start AND :end'

C KeyConditionExpression: 'PartitionKey = :pk AND SortKey BETWEEN :start AND :end'

D KeyConditionExpression: 'PartitionKey = :pk AND SortKey > :start AND SortKey < :end'

💡 Hint

Remember the correct operator to specify a range in DynamoDB queries.

The BETWEEN operator is used in KeyConditionExpression to specify a range for the sort key. AND is required to combine partition key equality and sort key range.

🔧 Debug

advanced

Identify the Error in Composite Key Query

Consider this DynamoDB query snippet:

KeyConditionExpression: 'PartitionKey = :pk AND SortKey = :sk'
ExpressionAttributeValues: { ':pk': 'User#789' }

What error will this cause?

A No error, query runs successfully

B Syntax error: Invalid KeyConditionExpression format

C Query returns no results because ':sk' is undefined

D Runtime error: Missing ExpressionAttributeValue for ':sk'

💡 Hint

Check if all placeholders in the expression have matching values.

The query uses ':sk' in the expression but does not provide a value for it in ExpressionAttributeValues, causing a runtime error.

❓ optimization

expert

Optimizing Access Patterns with Composite Primary Keys

You have a DynamoDB table with composite primary key (UserID, Timestamp). You want to efficiently retrieve the latest 5 records for a user. Which approach is best?

A Query with PartitionKey = UserID and SortKey in descending order, limit 5

B Scan the entire table and filter by UserID, then sort by Timestamp

C Use a Global Secondary Index on Timestamp only and filter by UserID

D Query with PartitionKey = UserID and SortKey in ascending order, limit 5

💡 Hint

Think about how DynamoDB stores items sorted by sort key within a partition.

Querying by PartitionKey and sorting SortKey descending allows efficient retrieval of latest records without scanning the whole table.

Practice

1.

What is a composite primary key in DynamoDB?

easy

A. A key that allows duplicate items

B. A key made of only one attribute

C. A key made of two attributes: PartitionKey and SortKey

D. A key used only for indexing

Solution

1. Step 1: Understand primary key types in DynamoDB

   DynamoDB supports simple primary keys (one attribute) and composite primary keys (two attributes).

2. Step 2: Identify composite primary key components

   A composite primary key consists of a PartitionKey and a SortKey to uniquely identify items.

3. Final Answer:

   A key made of two attributes: PartitionKey and SortKey -> Option C

4. Quick Check:

   Composite primary key = PartitionKey + SortKey [OK]

Hint: Composite key always has PartitionKey and SortKey [OK]

Common Mistakes:

• Thinking composite key has only one attribute
• Confusing composite key with secondary indexes
• Believing composite key allows duplicates

2.

Which of the following is the correct way to define a composite primary key in DynamoDB table creation?

{
  "TableName": "Orders",
  "KeySchema": [
    {"AttributeName": "CustomerId", "KeyType": "HASH"},
    {"AttributeName": "OrderDate", "KeyType": "RANGE"}
  ]
}

easy

A. Use KeyType RANGE for both attributes

B. Use KeyType RANGE for PartitionKey and HASH for SortKey

C. Use KeyType HASH for both attributes

D. Use KeyType HASH for PartitionKey and RANGE for SortKey

Solution

1. Step 1: Recall KeyType meanings

   In DynamoDB, HASH means PartitionKey and RANGE means SortKey.

2. Step 2: Match KeyType to attributes

   PartitionKey must be HASH and SortKey must be RANGE for composite keys.

3. Final Answer:

   Use KeyType HASH for PartitionKey and RANGE for SortKey -> Option D

4. Quick Check:

   PartitionKey=HASH, SortKey=RANGE [OK]

Hint: HASH = PartitionKey, RANGE = SortKey in KeySchema [OK]

Common Mistakes:

• Swapping HASH and RANGE roles
• Using same KeyType for both keys
• Confusing attribute names with KeyType

3.

Given a DynamoDB table with composite primary key (UserId as PartitionKey, Timestamp as SortKey), what will this query return?

{
  "TableName": "UserActivity",
  "KeyConditionExpression": "UserId = :uid and Timestamp > :time",
  "ExpressionAttributeValues": {
    ":uid": {"S": "user123"},
    ":time": {"N": "1609459200"}
  }
}

medium

A. All activities of user123 with Timestamp greater than 1609459200

B. All activities of all users with Timestamp greater than 1609459200

C. All activities of user123 regardless of Timestamp

D. Syntax error due to missing SortKey in query

Solution

1. Step 1: Understand KeyConditionExpression

   It filters items by PartitionKey equality and SortKey condition.

2. Step 2: Analyze the query conditions

   UserId = :uid restricts to user123; Timestamp > :time filters timestamps after 1609459200.

3. Final Answer:

   All activities of user123 with Timestamp greater than 1609459200 -> Option A

4. Quick Check:

   PartitionKey + SortKey condition filters items [OK]

Hint: PartitionKey = value AND SortKey condition filters items [OK]

Common Mistakes:

• Thinking query returns all users' data
• Ignoring SortKey condition
• Assuming syntax error without SortKey equality

4.

What is wrong with this DynamoDB query using composite primary key (UserId as PartitionKey, OrderId as SortKey)?

{
  "TableName": "Orders",
  "KeyConditionExpression": "OrderId = :oid",
  "ExpressionAttributeValues": {
    ":oid": {"S": "order789"}
  }
}

medium

A. PartitionKey UserId is missing in KeyConditionExpression

B. SortKey OrderId cannot be used in KeyConditionExpression

C. ExpressionAttributeValues syntax is incorrect

D. TableName is invalid

Solution

1. Step 1: Recall query requirements for composite keys

   Queries must specify PartitionKey equality in KeyConditionExpression.

2. Step 2: Check the given query

   Only SortKey OrderId is used; PartitionKey UserId is missing, causing error.

3. Final Answer:

   PartitionKey UserId is missing in KeyConditionExpression -> Option A

4. Quick Check:

   PartitionKey must be in query condition [OK]

Hint: Always include PartitionKey equality in query [OK]

Common Mistakes:

• Using only SortKey in query condition
• Assuming SortKey alone can identify items
• Ignoring required PartitionKey in queries

5.

You want to store blog posts in DynamoDB. Each post has a AuthorId and a PostDate. You want to quickly find all posts by an author sorted by date. Which composite primary key design is best?

hard

A. PartitionKey: PostDate, SortKey: AuthorId

B. PartitionKey: AuthorId, SortKey: PostDate

C. PartitionKey: AuthorId only, no SortKey

D. PartitionKey: PostDate only, no SortKey

Solution

1. Step 1: Identify query pattern

   You want to find all posts by an author, sorted by date.

2. Step 2: Choose PartitionKey and SortKey accordingly

   PartitionKey should be AuthorId to group posts by author; SortKey should be PostDate to sort posts by date.

3. Final Answer:

   PartitionKey: AuthorId, SortKey: PostDate -> Option B

4. Quick Check:

   Group by author, sort by date = AuthorId + PostDate [OK]

Hint: PartitionKey groups, SortKey sorts related items [OK]

Common Mistakes:

• Swapping PartitionKey and SortKey roles
• Using only one key losing sorting ability
• Choosing SortKey that doesn't support query pattern