Query by partition key in DynamoDB

Introduction

We use query by partition key to quickly find all items that share the same main identifier in a DynamoDB table.

When you want to get all orders made by a specific customer using their customer ID.

When you need to find all messages in a chat room identified by a room ID.

When you want to retrieve all products in a category identified by a category ID.

When you want to fetch all events happening on a specific date stored as a partition key.

Syntax

DynamoDB

aws dynamodb query \
  --table-name TableName \
  --key-condition-expression "PartitionKeyName = :partitionkeyval" \
  --expression-attribute-values '{":partitionkeyval":{"S":"PartitionKeyValue"}}'

The partition key is the main key that DynamoDB uses to organize data.

You must provide the partition key value to get matching items.

Examples

This finds all orders where the CustomerID is '12345'.

DynamoDB

aws dynamodb query \
  --table-name Orders \
  --key-condition-expression "CustomerID = :custid" \
  --expression-attribute-values '{":custid":{"S":"12345"}}'

This fetches all messages in the chat room with ID 'roomA'.

DynamoDB

aws dynamodb query \
  --table-name Messages \
  --key-condition-expression "RoomID = :roomid" \
  --expression-attribute-values '{":roomid":{"S":"roomA"}}'

Sample Program

This query gets all products in the 'electronics' category.

DynamoDB

aws dynamodb query \
  --table-name Products \
  --key-condition-expression "CategoryID = :catid" \
  --expression-attribute-values '{":catid":{"S":"electronics"}}'

Important Notes

Query by partition key is fast because DynamoDB uses the partition key to find data directly.

You cannot query by partition key alone if your table uses a composite key and you want to filter by sort key; you need to include conditions for the sort key too.

Summary

Query by partition key fetches all items with the same main key value.

It is efficient and fast because DynamoDB organizes data by partition key.

You provide the partition key value in the query to get matching items.

Practice

1. What does querying by partition key in DynamoDB do?

easy

A. Updates all items with the same partition key value

B. Deletes all items with the same partition key value

C. Fetches all items with the same partition key value

D. Creates a new item with the partition key value

Solution

1. Step 1: Understand partition key role

   The partition key uniquely identifies the partition where items are stored.

2. Step 2: Query by partition key fetches items

   Querying by partition key returns all items that share that key value.

3. Final Answer:

   Fetches all items with the same partition key value -> Option C

4. Quick Check:

   Query by partition key = fetch items [OK]

Hint: Partition key query fetches matching items only [OK]

Common Mistakes:

• Thinking it deletes or updates items
• Confusing partition key with sort key
• Assuming it creates new items

2. Which of the following is the correct syntax to query items by partition key 'UserID' with value '123' in DynamoDB?

easy

A. Table.query(KeyConditionExpression: 'UserID = 123')

B. Table.query(KeyConditionExpression: 'UserID = :val', ExpressionAttributeValues: {':val': '123'})

C. Table.query(FilterExpression: 'UserID = 123')

D. Table.query(KeyConditionExpression: 'UserID == 123')

Solution

1. Step 1: Identify correct query syntax

   DynamoDB requires KeyConditionExpression with placeholders and ExpressionAttributeValues.

2. Step 2: Check each option

   Table.query(KeyConditionExpression: 'UserID = :val', ExpressionAttributeValues: {':val': '123'}) uses correct syntax with placeholder ':val' and value mapping. Others misuse operators or omit placeholders.

3. Final Answer:

   Table.query(KeyConditionExpression: 'UserID = :val', ExpressionAttributeValues: {':val': '123'}) -> Option B

4. Quick Check:

   Use placeholders and ExpressionAttributeValues [OK]

Hint: Use placeholders (:val) and ExpressionAttributeValues [OK]

Common Mistakes:

• Using '==' instead of '=' in KeyConditionExpression
• Not using ExpressionAttributeValues for values
• Using FilterExpression instead of KeyConditionExpression

3. Given a DynamoDB table with items: {UserID: '123', Name: 'Alice'}, {UserID: '123', Name: 'Bob'}, {UserID: '456', Name: 'Carol'}, what will the query by partition key 'UserID' = '123' return?

medium

A. [{UserID: '123', Name: 'Alice'}, {UserID: '123', Name: 'Bob'}]

B. [{UserID: '456', Name: 'Carol'}]

C. [{UserID: '123', Name: 'Alice'}]

D. [] (empty list)

Solution

1. Step 1: Identify items with partition key '123'

   Items with UserID '123' are Alice and Bob.

2. Step 2: Query returns all matching items

   Query returns both Alice and Bob items as they share the partition key.

3. Final Answer:

   [{UserID: '123', Name: 'Alice'}, {UserID: '123', Name: 'Bob'}] -> Option A

4. Quick Check:

   Query by '123' returns Alice and Bob [OK]

Hint: Query returns all items with matching partition key [OK]

Common Mistakes:

• Expecting only one item returned
• Confusing partition key with sort key filtering
• Assuming query returns items with different partition keys

4. You wrote this query to get items with partition key 'OrderID' = '789': Table.query(KeyConditionExpression: 'OrderID = 789'). It returns an error. What is the problem?

medium

A. Partition key name is case-sensitive and should be 'orderid'

B. Using wrong operator '==' instead of '='

C. Query method does not support KeyConditionExpression

D. Missing ExpressionAttributeValues for the value '789'

Solution

1. Step 1: Check query syntax requirements

   DynamoDB requires placeholders and ExpressionAttributeValues for values in KeyConditionExpression.

2. Step 2: Identify missing part

   The query uses 'OrderID = 789' directly without placeholder or ExpressionAttributeValues, causing error.

3. Final Answer:

   Missing ExpressionAttributeValues for the value '789' -> Option D

4. Quick Check:

   Use placeholders and ExpressionAttributeValues [OK]

Hint: Always use placeholders and ExpressionAttributeValues [OK]

Common Mistakes:

• Using raw values instead of placeholders
• Assuming case-insensitivity for partition key
• Using '==' operator instead of '='

5. You want to query a DynamoDB table for all items with partition key 'Category' = 'Books' and sort key 'Price' less than 20. Which approach correctly uses query by partition key with a condition on sort key?

hard

A. Use KeyConditionExpression: 'Category = :cat AND Price < :price' with ExpressionAttributeValues {':cat': 'Books', ':price': 20}

B. Use FilterExpression: 'Category = :cat AND Price < :price' with ExpressionAttributeValues {':cat': 'Books', ':price': 20}

C. Use KeyConditionExpression: 'Category = :cat' and FilterExpression: 'Price < :price' with ExpressionAttributeValues {':cat': 'Books', ':price': 20}

D. Use FilterExpression: 'Category = :cat' and KeyConditionExpression: 'Price < :price' with ExpressionAttributeValues {':cat': 'Books', ':price': 20}

Solution

1. Step 1: Understand query conditions

   KeyConditionExpression must include partition key equality and supports sort key conditions like <. FilterExpression filters results after the query.

2. Step 2: Analyze options

   Use KeyConditionExpression: 'Category = :cat AND Price < :price' with ExpressionAttributeValues {':cat': 'Books', ':price': 20} correctly uses KeyConditionExpression: 'Category = :cat AND Price < :price', which efficiently queries the partition with a sort key range condition. Use KeyConditionExpression: 'Category = :cat' and FilterExpression: 'Price < :price' with ExpressionAttributeValues {':cat': 'Books', ':price': 20} uses FilterExpression for Price (valid but less efficient). B lacks KeyConditionExpression. D omits partition key from KeyConditionExpression.

3. Final Answer:

   Use KeyConditionExpression: 'Category = :cat AND Price < :price' with ExpressionAttributeValues {':cat': 'Books', ':price': 20} -> Option A

4. Quick Check:

   KeyConditionExpression: partition = AND sort < [OK]

Hint: KeyConditionExpression: partition = AND sort condition [OK]

Common Mistakes:

• Using FilterExpression for partition key
• Using FilterExpression instead of KeyConditionExpression for sort key range
• Mixing KeyConditionExpression and FilterExpression incorrectly