How to join two tables with primary key using dynamodb - amazon-dynamodb

Question

I am new to DynamoDB. I have two tables:
country
city
I wish to join both tables via the country_id primary key and foreign key. So can I do this in DynamoDB?

Answer 1

Amazon DynamoDB is a NoSQL database. This means that traditional relational database concepts such as JOINs and Foreign Keys are not available.
Your application would be responsible for "joining" tables. That is, you would need to read values from both tables and determine a relationship between them within your application. DynamoDB is not able to do this for you.
Alternatively, you could use a system such as Amazon EMR, which provides Hadoop. You could use Hadoop and Hive to access a DynamoDB table by using HiveQL (which is similar to SQL). However, this is a very slow operation and might be too complex for your particular requirement.

Answer 2

As #John said, Amazon DynamoDB is a NoSQL database. So it's impossible do this at one query. But, Of course, you can get the data through multiple queries if you want. Below is dynamoDB table scheme.(It's also the table creation code)
import { CreateTableInput } from 'aws-sdk/clients/dynamodb';
import AWS from 'aws-sdk'
const paramCountryTable: CreateTableInput = {
BillingMode: 'PAY_PER_REQUEST', // for just example
TableName: 'Country',
AttributeDefinitions: [
{ AttributeName: 'country_id', AttributeType: 'S' },
],
KeySchema: [
{ AttributeName: 'country_id', KeyType: 'HASH' },
],
}
const paramCityTable: CreateTableInput = {
BillingMode: 'PAY_PER_REQUEST', // for just example
TableName: 'City',
AttributeDefinitions: [
{ AttributeName: 'city_id', AttributeType: 'S' },
{ AttributeName: 'country_id', AttributeType: 'S' },
],
KeySchema: [
{ AttributeName: 'city_id', KeyType: 'HASH' },
],
GlobalSecondaryIndexes: [{
Projection: { ProjectionType: 'ALL' },
IndexName: 'IndexCountryId',
KeySchema: [
{ AttributeName: 'country_id', KeyType: 'HASH' },
],
}],
}
const run = async () => {
const dd = new AWS.DynamoDB({ apiVersion: '2012-08-10' })
await dd.createTable(paramCountryTable).promise()
await dd.createTable(paramCityTable).promise()
}
if (require.main === module) run()
Then, you can implement what you want through several queries as below.
const ddc = new AWS.DynamoDB.DocumentClient({ apiVersion: '2012-08-10' })
const getCountryByCityId = async (city_id: string) => {
const resultCity = await ddc.get({ TableName: 'City', Key: {city_id}}).promise()
if (!resultCity.Item) return null
const { country_id } = resultCity.Item
const resultCountry = await ddc.get({ TableName: 'Country', Key: {country_id}}).promise()
return resultCountry.Item
}
const getCityArrByCountryId = async (country_id: string) => {
const result = await ddc.query({
TableName: 'City',
IndexName: 'IndexCountryId',
ExpressionAttributeValues: { ":country_id": country_id },
KeyConditionExpression: "country_id = :country_id",
}).promise()
return result.Items
}
Note that this is not a recommended use case for NoSQL.