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How to compute the difference between two columns with ksqlDB

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How to compute the difference between two columns with ksqlDB

Suppose you have a topic with events that represent a customer's purchase history, where each event also includes the amount of the customer's previous purchase. In this tutorial, we will use ksqlDB to calculate the difference between the current and previous purchase amounts.

Setup

Assume a stream named customer_purchases in which each event contains the amounts of the customer's current and previous purchases:

CREATE STREAM customer_purchases (
        id VARCHAR,
        current_purchase DOUBLE,
        previous_purchase DOUBLE,
        txn_ts VARCHAR,
        first_name VARCHAR,
        last_name  VARCHAR)
    WITH (KAFKA_TOPIC='customer_purchases',
          VALUE_FORMAT='JSON',
          PARTITIONS=1);

Calculate the difference between two columns

Now create a query to determine the difference between two columns:

SELECT first_name,
       last_name,
       current_purchase - previous_purchase as purchase_diff
FROM customer_purchases
EMIT CHANGES;

Running the example

You can run the example backing this tutorial in one of two ways: locally with the ksql CLI against Kafka and ksqlDB running in Docker, or with Confluent Cloud.

Local With Docker

Prerequisites

Run the commands

Clone the confluentinc/tutorials GitHub repository (if you haven't already) and navigate to the tutorials directory:

git clone git@github.com:confluentinc/tutorials.git
cd tutorials

Start ksqlDB and Kafka:

docker compose -f ./docker/docker-compose-ksqldb.yml up -d

Next, open the ksqlDB CLI:

docker exec -it ksqldb-cli ksql http://ksqldb-server:8088

Run the following SQL statements to create the customer_purchases stream backed by Kafka running in Docker and populate it with test data.

CREATE STREAM customer_purchases (
        current_purchase DOUBLE,
        previous_purchase DOUBLE,
        txn_ts VARCHAR,
        first_name VARCHAR,
        last_name  VARCHAR)
    WITH (KAFKA_TOPIC='customer-purchases',
          VALUE_FORMAT='JSON',
          PARTITIONS=1);
INSERT INTO customer_purchases (current_purchase, previous_purchase, txn_ts, first_name, last_name)
    VALUES (50.89, 28.99, '2024-08-04 02:35:43', 'Tony', 'Stark');
INSERT INTO customer_purchases (current_purchase, previous_purchase, txn_ts, first_name, last_name)
    VALUES (23.12, 12.99, '2024-08-04 02:35:44', 'Nick', 'Fury');
INSERT INTO customer_purchases (current_purchase, previous_purchase, txn_ts, first_name, last_name)
    VALUES (20.00, 6.99, '2024-08-04 02:35:45', 'Natasha', 'Romanov');
INSERT INTO customer_purchases (current_purchase, previous_purchase, txn_ts, first_name, last_name)
    VALUES (99.29, 1.99, '2024-08-04 02:35:46', 'Wanda', 'Maximoff');

Finally, run the column difference query. Note that we first tell ksqlDB to consume from the beginning of the stream.

SET 'auto.offset.reset'='earliest';

SELECT first_name,
       last_name,
       current_purchase - previous_purchase as purchase_diff
FROM customer_purchases
EMIT CHANGES;

The query output should look like this:

+-------------------+-------------------+-------------------+
|FIRST_NAME         |LAST_NAME          |PURCHASE_DIFF      |
+-------------------+-------------------+-------------------+
|Tony               |Stark              |21.90              |
|Nick               |Fury               |10.13              |
|Natasha            |Romanov            |13.01              |
|Wanda              |Maximoff           |97.30              |
+-------------------+-------------------+-------------------+

When you are finished, exit the ksqlDB CLI by entering CTRL-D and clean up the containers used for this tutorial by running:

docker compose -f ./docker/docker-compose-ksqldb.yml down
Confluent Cloud

Prerequisites

Create Confluent Cloud resources

Login to your Confluent Cloud account:

confluent login --prompt --save

Install a CLI plugin that will streamline the creation of resources in Confluent Cloud:

confluent plugin install confluent-cloud_kickstart

Run the following command to create a Confluent Cloud environment and Kafka cluster. This will create resources in AWS region us-west-2 by default, but you may override these choices by passing the --cloud argument with a value of aws, gcp, or azure, and the --region argument that is one of the cloud provider's supported regions, which you can list by running confluent kafka region list --cloud <CLOUD PROVIDER>

confluent cloud-kickstart --name ksqldb-tutorial \
  --environment-name ksqldb-tutorial \
  --output-format stdout

Now, create a ksqlDB cluster by first getting your user ID of the form u-123456 when you run this command:

confluent iam user list

And then create a ksqlDB cluster called ksqldb-tutorial with access linked to your user account:

confluent ksql cluster create ksqldb-tutorial \
  --credential-identity <USER ID>

Run the commands

Login to the Confluent Cloud Console. Select Environments in the lefthand navigation, and then click the ksqldb-tutorial environment tile. Click the ksqldb-tutorial Kafka cluster tile, and then select ksqlDB in the lefthand navigation.

The cluster may take a few minutes to be provisioned. Once its status is Up, click the cluster name and scroll down to the editor.

In the query properties section at the bottom, change the value for auto.offset.reset to Earliest so that ksqlDB will consume from the beginning of the stream we create.

Enter the following statements in the editor and click Run query. This creates the customer_purchases stream and populates it with test data.

CREATE STREAM customer_purchases (
        current_purchase DOUBLE,
        previous_purchase DOUBLE,
        txn_ts VARCHAR,
        first_name VARCHAR,
        last_name  VARCHAR)
    WITH (KAFKA_TOPIC='customer-purchases',
          VALUE_FORMAT='JSON',
          PARTITIONS=1);

INSERT INTO customer_purchases (current_purchase, previous_purchase, txn_ts, first_name, last_name)
    VALUES (50.89, 28.99, '2024-08-04 02:35:43', 'Tony', 'Stark');
INSERT INTO customer_purchases (current_purchase, previous_purchase, txn_ts, first_name, last_name)
    VALUES (23.12, 12.99, '2024-08-04 02:35:44', 'Nick', 'Fury');
INSERT INTO customer_purchases (current_purchase, previous_purchase, txn_ts, first_name, last_name)
    VALUES (20.00, 6.99, '2024-08-04 02:35:45', 'Natasha', 'Romanov');
INSERT INTO customer_purchases (current_purchase, previous_purchase, txn_ts, first_name, last_name)
    VALUES (99.29, 1.99, '2024-08-04 02:35:46', 'Wanda', 'Maximoff');

Now paste the query to calculate the column difference in the editor and click Run query:

SET 'auto.offset.reset'='earliest';

SELECT first_name,
       last_name,
       current_purchase - previous_purchase as purchase_diff
FROM customer_purchases
EMIT CHANGES;

The query output should look like this:

+-------------------+-------------------+-------------------+
|FIRST_NAME         |LAST_NAME          |PURCHASE_DIFF      |
+-------------------+-------------------+-------------------+
|Tony               |Stark              |21.90              |
|Nick               |Fury               |10.13              |
|Natasha            |Romanov            |13.01              |
|Wanda              |Maximoff           |97.30              |
+-------------------+-------------------+-------------------+

Clean up

When you are finished, delete the ksqldb-tutorial environment by first getting the environment ID of the form env-123456 corresponding to it:

confluent environment list

Delete the environment, including all resources created for this tutorial:

confluent environment delete <ENVIRONMENT ID>
Do you have questions or comments? Join us in the #confluent-developer community Slack channel to engage in discussions with the creators of this content.