Menu
Amazon DynamoDB
Developer Guide (API Version 2012-08-10)

Step 4: Query and Scan the Data

You can use the query method to retrieve data from a table. You must specify a partition key value; the sort key is optional.

The primary key for the Movies table is composed of the following:

  • year – The partition key. The attribute type is number. 

  • title – The sort key. The attribute type is string.

To find all movies released during a year, you need to specify only the year. You can also provide the title to retrieve a subset of movies based on some condition (on the sort key); for example, to find movies released in 2014 that have a title starting with the letter "A".

In addition to query, there is also a scan method that can retrieve all the table data.

To learn more about querying and scanning data, see Working with Queries and Working with Scans, respectively.

Step 4.1: Query - All Movies Released in a Year

The program included in this step retrieves all movies released in the year 1985.

  1. Copy and paste the following program into a file named MoviesQuery01.js:

    Copy
    var AWS = require("aws-sdk"); AWS.config.update({ region: "us-west-2", endpoint: "http://localhost:8000" }); var docClient = new AWS.DynamoDB.DocumentClient(); console.log("Querying for movies from 1985."); var params = { TableName : "Movies", KeyConditionExpression: "#yr = :yyyy", ExpressionAttributeNames:{ "#yr": "year" }, ExpressionAttributeValues: { ":yyyy":1985 } }; docClient.query(params, function(err, data) { if (err) { console.error("Unable to query. Error:", JSON.stringify(err, null, 2)); } else { console.log("Query succeeded."); data.Items.forEach(function(item) { console.log(" -", item.year + ": " + item.title); }); } });

    Note

    ExpressionAttributeNames provides name substitution. We use this because year is a reserved word in DynamoDB—you cannot use it directly in any expression, including KeyConditionExpression. We use the expression attribute name #yr to address this.

    ExpressionAttributeValues provides value substitution. We use this because you cannot use literals in any expression, including KeyConditionExpression. We use the expression attribute value :yyyy to address this.

  2. To run the program, type the following command:

    node MoviesQuery01.js

Note

The preceding program shows how to query a table by its primary key attributes. In DynamoDB, you can optionally create one or more secondary indexes on a table, and query those indexes in the same way that you query a table. Secondary indexes give your applications additional flexibility by allowing queries on non-key attributes. For more information, see Improving Data Access with Secondary Indexes.

Step 4.2: Query - All Movies Released in a Year with Certain Titles

The program included in this step retrieves all movies released in year 1992, with title beginning with the letter "A" through the letter "L".

  1. Copy and paste the following program into a file named MoviesQuery02.js:

    Copy
    var AWS = require("aws-sdk"); AWS.config.update({ region: "us-west-2", endpoint: "http://localhost:8000" }); var docClient = new AWS.DynamoDB.DocumentClient(); console.log("Querying for movies from 1992 - titles A-L, with genres and lead actor"); var params = { TableName : "Movies", ProjectionExpression:"#yr, title, info.genres, info.actors[0]", KeyConditionExpression: "#yr = :yyyy and title between :letter1 and :letter2", ExpressionAttributeNames:{ "#yr": "year" }, ExpressionAttributeValues: { ":yyyy":1992, ":letter1": "A", ":letter2": "L" } }; docClient.query(params, function(err, data) { if (err) { console.log("Unable to query. Error:", JSON.stringify(err, null, 2)); } else { console.log("Query succeeded."); data.Items.forEach(function(item) { console.log(" -", item.year + ": " + item.title + " ... " + item.info.genres + " ... " + item.info.actors[0]); }); } });
  2. To run the program, type the following command:

    node MoviesQuery02.js

Step 4.3: Scan

The scan method reads every item in the table, and returns all the data in the table. You can provide an optional filter_expression, so that only the items matching your criteria are returned. However, the filter is only applied after the entire table has been scanned.

The following program scans the entire Movies table, which contains approximately 5,000 items. The scan specifies the optional filter to retrieve only the movies from the 1950s (approximately 100 items), and discard all of the others.

  1. Copy and paste the following program into a file named MoviesScan.js:

    Copy
    var AWS = require("aws-sdk"); AWS.config.update({ region: "us-west-2", endpoint: "http://localhost:8000" }); var docClient = new AWS.DynamoDB.DocumentClient(); var params = { TableName: "Movies", ProjectionExpression: "#yr, title, info.rating", FilterExpression: "#yr between :start_yr and :end_yr", ExpressionAttributeNames: { "#yr": "year", }, ExpressionAttributeValues: { ":start_yr": 1950, ":end_yr": 1959 } }; console.log("Scanning Movies table."); docClient.scan(params, onScan); function onScan(err, data) { if (err) { console.error("Unable to scan the table. Error JSON:", JSON.stringify(err, null, 2)); } else { // print all the movies console.log("Scan succeeded."); data.Items.forEach(function(movie) { console.log( movie.year + ": ", movie.title, "- rating:", movie.info.rating); }); // continue scanning if we have more movies, because // scan can retrieve a maximum of 1MB of data if (typeof data.LastEvaluatedKey != "undefined") { console.log("Scanning for more..."); params.ExclusiveStartKey = data.LastEvaluatedKey; docClient.scan(params, onScan); } } }

    In the code, note the following:

    • ProjectionExpression specifies the attributes you want in the scan result.

    • FilterExpression specifies a condition that returns only items that satisfy the condition. All other items are discarded.

  2. To run the program, type the following command:

    node MoviesScan.js

Note

You can also use the Scan operation with any secondary indexes that you have created on the table. For more information, see Improving Data Access with Secondary Indexes.