[20 Days of DynamoDB] Day 9 - Query API

Posted - 18/Jan/2024

The Query API is used to model one-to-many relationships in DynamoDB. You can search for items based on (composite) primary key values using Key Condition Expressions. The value for partition key attribute is mandatory - the query returns all items with that partition key value. Additionally, you can also provide a sort key attribute and use a comparison operator to refine the search results.

With the Query API, you can also:

1. Switch to strongly consistent read (eventual consistent being the default)
2. Use a projection expression to return only some attributes
3. Return the consumed Read Capacity Units (RCU)

Here is an example that queries for a specific thread based on the forum name (partition key) and subject (sort key). It only returns the Message attribute:

resp,err=client.Query(context.Background(),&dynamodb.QueryInput{TableName:aws.String(tableName),KeyConditionExpression:aws.String("ForumName = :name and Subject = :sub"),ExpressionAttributeValues:map[string]types.AttributeValue{":name":&types.AttributeValueMemberS{Value:forumName},":sub":&types.AttributeValueMemberS{Value:subject},},ReturnConsumedCapacity:types.ReturnConsumedCapacityTotal,ConsistentRead:aws.Bool(true),ProjectionExpression:aws.String("Message"),})

Recommended reading:

1. API documentation
2. Item Collections
3. Key Condition Expressions
4. Composite primary key