On a previous post we proceeded on inserting data on a DynamoDB database.

On this tutorial we will issue some basic queries against our DynamoDB tables. The main rule is that every query has to use the hash key.

The simplest form of query is using the hash key only. We will query the Users table on this one. There would be only one result, therefore there is no use on iterating the Items list.

public Map<String,AttributeValue> getUser(String email) {

 Map<String,String> expressionAttributesNames = new HashMap<>();
 expressionAttributesNames.put("#email","email");

 Map<String,AttributeValue> expressionAttributeValues = new HashMap<>();
 expressionAttributeValues.put(":emailValue",new AttributeValue().withS(email));

 QueryRequest queryRequest = new QueryRequest()
 .withTableName(TABLE_NAME)
 .withKeyConditionExpression("#email = :emailValue")
 .withExpressionAttributeNames(expressionAttributesNames)
 .withExpressionAttributeValues(expressionAttributeValues);

 QueryResult queryResult = amazonDynamoDB.query(queryRequest);

 List<Map<String,AttributeValue>> attributeValues = queryResult.getItems();

 if(attributeValues.size()>0) {
 return attributeValues.get(0);
 } else {
 return null;
 }
 }

However we can issue more complex queries using conditions.
Logins Table suits well for an example. We will issue a query that will fetch login attempts between to dates.

public List<Map<String ,AttributeValue>> queryLoginsBetween(String email, Date from, Date to) {

 List<Map<String,AttributeValue>> items = new ArrayList<>();

 Map<String,String> expressionAttributesNames = new HashMap<>();
 expressionAttributesNames.put("#email","email");
 expressionAttributesNames.put("#timestamp","timestamp");

 Map<String,AttributeValue> expressionAttributeValues = new HashMap<>();
 expressionAttributeValues.put(":emailValue",new AttributeValue().withS(email));
 expressionAttributeValues.put(":from",new AttributeValue().withN(Long.toString(from.getTime())));
 expressionAttributeValues.put(":to",new AttributeValue().withN(Long.toString(to.getTime())));

 QueryRequest queryRequest = new QueryRequest()
 .withTableName(TABLE_NAME)
 .withKeyConditionExpression("#email = :emailValue and #timestamp BETWEEN :from AND :to ")
 .withExpressionAttributeNames(expressionAttributesNames)
 .withExpressionAttributeValues(expressionAttributeValues);

 Map<String,AttributeValue> lastKey = null;

 do {

 QueryResult queryResult = amazonDynamoDB.query(queryRequest);
 List<Map<String,AttributeValue>> results = queryResult.getItems();
 items.addAll(results);
 lastKey = queryResult.getLastEvaluatedKey();
 } while (lastKey!=null);

 return items;
 }

Keep in mind that DynamoDB Fetches data in pages, therefore you have to issue the same request more than once in case of multiple pages. Therefore you have to use the last evaluated key to your next request.

Last but not least querying on indexes is one of the basic actions. It is the same routine either for local or global secondary indexes.
Keep in mind that the results fetched depend on the projection type we specified once creating the Table. In our case the projection type is for all fields.

We shall use the Supervisors table.

public Map<String ,AttributeValue> getSupervisor(String company,String factory) {

 List<Map<String,AttributeValue>> items = new ArrayList<>();

 Map<String,String> expressionAttributesNames = new HashMap<>();
 expressionAttributesNames.put("#company","company");
 expressionAttributesNames.put("#factory","factory");

 Map<String,AttributeValue> expressionAttributeValues = new HashMap<>();
 expressionAttributeValues.put(":company",new AttributeValue().withS(company));
 expressionAttributeValues.put(":factory",new AttributeValue().withS(factory));

 QueryRequest queryRequest = new QueryRequest()
 .withTableName(TABLE_NAME)
 .withKeyConditionExpression("#company = :company and #factory = :factory ")
 .withIndexName("FactoryIndex")
 .withExpressionAttributeNames(expressionAttributesNames)
 .withExpressionAttributeValues(expressionAttributeValues);

 QueryResult queryResult = amazonDynamoDB.query(queryRequest);

 List<Map<String,AttributeValue>> attributeValues = queryResult.getItems();

 if(attributeValues.size()>0) {
 return attributeValues.get(0);
 } else {
 return null;
 }
 }

You can find full source code with unit tests on github.