In a previous post, we tackled inserting data into a DynamoDB database.

In 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 iterating the Items list.

var getUser = function(email,callback) {

    var docClient = new AWS.DynamoDB.DocumentClient();

    var params = {
        TableName: "Users",
        KeyConditionExpression: "#email = :email",
        ExpressionAttributeNames:{
            "#email": "email"
            },
        ExpressionAttributeValues: {
            ":email":email
            }
        };

    docClient.query(params,callback);
    };

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

var queryLogins = function(email,from,to,callback) {

    var docClient = new AWS.DynamoDB.DocumentClient();

    var params = {
        TableName:"Logins",
        KeyConditionExpression:"#email = :emailValue and #timestamp BETWEEN :from AND :to",
        ExpressionAttributeNames: {
            "#email":"email",
            "#timestamp":"timestamp"
            },
        ExpressionAttributeValues: {
            ":emailValue":email,
            ":from": from.getTime(),
            ":to":to.getTime()
            }
        };

    var items = []

    var queryExecute = function(callback) {

        docClient.query(params,function(err,result) {

            if(err) {
                callback(err);
                } else {

                console.log(result)

                items = items.concat(result.Items);

                if(result.LastEvaluatedKey) {

                    params.ExclusiveStartKey = result.LastEvaluatedKey;
                    queryExecute(callback);
                    } else {
                        callback(err,items);
                    }
                }
            });
        }

        queryExecute(callback);
    };

Keep in mind that DynamoDB fetches data in pages, therefore you have to issue the same request more than once in the case of multiple pages. You have to use the last evaluated key to your next request. If there are a lot of entries, be aware that you should handle the call stack size.

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.

var docClient = new AWS.DynamoDB.DocumentClient();

var params = {
    TableName: "Supervisors",
    IndexName: "FactoryIndex",
    KeyConditionExpression:"#company = :companyValue and #factory = :factoryValue",
    ExpressionAttributeNames: {
        "#company":"company",
        "#factory":"factory"
        },
    ExpressionAttributeValues: {
        ":companyValue": company,
        ":factoryValue": factory
        }
    };

docClient.query(params,callback);

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