Reading and Writing Data

DynamoDB provides a rich set of API operations for reading and writing data. This lesson covers the core operations — PutItem, GetItem, UpdateItem, DeleteItem, Query, and Scan — along with conditional expressions, batch operations, and transactions.

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Write Operations

PutItem

Creates a new item or replaces an existing item with the same primary key:

aws dynamodb put-item \
  --table-name Users \
  --item '{
    "UserId": {"S": "user-001"},
    "Name": {"S": "Alice"},
    "Email": {"S": "alice@example.com"},
    "Age": {"N": "30"}
  }'

Important: PutItem replaces the entire item. If an item with the same key exists, all attributes are overwritten.

UpdateItem

Modifies specific attributes of an existing item (or creates it if it does not exist):

aws dynamodb update-item \
  --table-name Users \
  --key '{"UserId": {"S": "user-001"}}' \
  --update-expression "SET Age = :newAge, UpdatedAt = :ts" \
  --expression-attribute-values '{
    ":newAge": {"N": "31"},
    ":ts": {"S": "2024-03-04T12:00:00Z"}
  }'

Update Expression Actions

Action	Description	Example
SET	Add or update attributes	SET Name = :name
REMOVE	Delete attributes	REMOVE OldField
ADD	Increment numbers or add to sets	ADD ViewCount :inc
DELETE	Remove elements from sets	DELETE Tags :tagsToRemove

DeleteItem

Deletes an item by its primary key:

aws dynamodb delete-item \
  --table-name Users \
  --key '{"UserId": {"S": "user-001"}}'

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Read Operations

GetItem

Retrieves a single item by its full primary key:

aws dynamodb get-item \
  --table-name Users \
  --key '{"UserId": {"S": "user-001"}}' \
  --consistent-read

Parameter	Description
--consistent-read	Strongly consistent read (default is eventually consistent)
--projection-expression	Return only specific attributes

Query

Retrieves multiple items from a single partition using the partition key and optional sort key conditions:

aws dynamodb query \
  --table-name Orders \
  --key-condition-expression "CustomerId = :cid AND OrderDate BETWEEN :start AND :end" \
  --expression-attribute-values '{
    ":cid": {"S": "cust-001"},
    ":start": {"S": "2024-01-01"},
    ":end": {"S": "2024-03-31"}
  }'

Key points about Query:

• You must specify the partition key with an equality condition
• The sort key condition is optional and supports =, <, <=, >, >=, BETWEEN, begins_with
• Results are sorted by the sort key (ascending by default; use --scan-index-forward false for descending)
• Returns up to 1 MB of data per call (use pagination for more)

Scan

Reads every item in a table:

aws dynamodb scan \
  --table-name Users \
  --filter-expression "Age > :minAge" \
  --expression-attribute-values '{":minAge": {"N": "25"}}'

Warning: Scan reads the entire table and then applies the filter. It consumes read capacity for every item examined, not just those returned. Avoid Scan in production whenever possible — use Query or indexes instead.

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Conditional Expressions

Add conditions to write operations to implement optimistic locking and business rules: