Column Difference

Question:

How can you calculate the difference between two columns?

Example use case:

Suppose you have a table or stream and you need to calculate the difference between two columns. In this tutorial, we'll show how to calculate the difference between two columns.

Hands-on code example:

ksqlDB

Short Answer

Take the fields you want to calculate the difference between and use the - operator between them field_1 - field_2

SELECT FIRST_NAME,
       LAST_NAME,
       CURRENT_PURCHASE - PREVIOUS_PURCHASE as PURCHASE_DIFF

Note that the - operator expects numerical values. So if have columns where the numbers are stored as VARCHAR you’ll have to use a CAST operation to convert them to a numerical type, otherwise you’ll get an error in your query.

Run it

Prerequisites

This tutorial installs Confluent Platform using Docker. Before proceeding:

• Install Docker Desktop (version 4.0.0 or later) or Docker Engine (version 19.03.0 or later) if you don’t already have it
• Install the Docker Compose plugin if you don’t already have it. This isn’t necessary if you have Docker Desktop since it includes Docker Compose.
• Start Docker if it’s not already running, either by starting Docker Desktop or, if you manage Docker Engine with systemd, via systemctl
• Verify that Docker is set up properly by ensuring no errors are output when you run docker info and docker compose version on the command line

Initialize the project

To get started, make a new directory anywhere you’d like for this project:

mkdir column-difference && cd column-difference

Then make the following directories to set up its structure:

mkdir src test

Get Confluent Platform

Next, create the following docker-compose.yml file to obtain Confluent Platform (for Kafka in the cloud, see Confluent Cloud):

version: '2'
services:
  broker:
    image: confluentinc/cp-kafka:7.4.1
    hostname: broker
    container_name: broker
    ports:
    - 29092:29092
    environment:
      KAFKA_BROKER_ID: 1
      KAFKA_LISTENER_SECURITY_PROTOCOL_MAP: PLAINTEXT:PLAINTEXT,PLAINTEXT_HOST:PLAINTEXT,CONTROLLER:PLAINTEXT
      KAFKA_ADVERTISED_LISTENERS: PLAINTEXT://broker:9092,PLAINTEXT_HOST://localhost:29092
      KAFKA_OFFSETS_TOPIC_REPLICATION_FACTOR: 1
      KAFKA_GROUP_INITIAL_REBALANCE_DELAY_MS: 0
      KAFKA_TRANSACTION_STATE_LOG_MIN_ISR: 1
      KAFKA_TRANSACTION_STATE_LOG_REPLICATION_FACTOR: 1
      KAFKA_PROCESS_ROLES: broker,controller
      KAFKA_NODE_ID: 1
      KAFKA_CONTROLLER_QUORUM_VOTERS: 1@broker:29093
      KAFKA_LISTENERS: PLAINTEXT://broker:9092,CONTROLLER://broker:29093,PLAINTEXT_HOST://0.0.0.0:29092
      KAFKA_INTER_BROKER_LISTENER_NAME: PLAINTEXT
      KAFKA_CONTROLLER_LISTENER_NAMES: CONTROLLER
      KAFKA_LOG_DIRS: /tmp/kraft-combined-logs
      CLUSTER_ID: MkU3OEVBNTcwNTJENDM2Qk
  schema-registry:
    image: confluentinc/cp-schema-registry:7.3.0
    hostname: schema-registry
    container_name: schema-registry
    depends_on:
    - broker
    ports:
    - 8081:8081
    environment:
      SCHEMA_REGISTRY_HOST_NAME: schema-registry
      SCHEMA_REGISTRY_KAFKASTORE_BOOTSTRAP_SERVERS: broker:9092
  ksqldb-server:
    image: confluentinc/ksqldb-server:0.28.2
    hostname: ksqldb-server
    container_name: ksqldb-server
    depends_on:
    - broker
    - schema-registry
    ports:
    - 8088:8088
    environment:
      KSQL_CONFIG_DIR: /etc/ksqldb
      KSQL_LOG4J_OPTS: -Dlog4j.configuration=file:/etc/ksqldb/log4j.properties
      KSQL_BOOTSTRAP_SERVERS: broker:9092
      KSQL_HOST_NAME: ksqldb-server
      KSQL_LISTENERS: http://0.0.0.0:8088
      KSQL_CACHE_MAX_BYTES_BUFFERING: 0
      KSQL_KSQL_SCHEMA_REGISTRY_URL: http://schema-registry:8081
      KSQL_KSQL_STREAMS_AUTO_OFFSET_RESET: earliest
  ksqldb-cli:
    image: confluentinc/ksqldb-cli:0.28.2
    container_name: ksqldb-cli
    depends_on:
    - broker
    - ksqldb-server
    entrypoint: /bin/sh
    tty: true
    environment:
      KSQL_CONFIG_DIR: /etc/ksqldb
    volumes:
    - ./src:/opt/app/src
    - ./test:/opt/app/test

And launch it by running:

docker compose up -d

Create the ksqlDB stream interactively using the CLI

To begin developing interactively, open up the ksqlDB CLI:

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

The first thing we do is to create a stream named PURCHASE_STREAM. This statement creates the customer_purchases topic, since it doesn’t already exist. For more details check out the ksqlDB documentation on the CREATE STREAM statement. The data contained in the topic is just plain, schemaless JSON.

CREATE STREAM PURCHASE_STREAM (
        	      ID VARCHAR,
                      PREVIOUS_PURCHASE DOUBLE,
                      CURRENT_PURCHASE DOUBLE,
                      TXN_TS VARCHAR,
                      FIRST_NAME VARCHAR,
                      LAST_NAME  VARCHAR)

 WITH (KAFKA_TOPIC='customer_purchases',
       VALUE_FORMAT='JSON',
       PARTITIONS=1);

Go ahead and create the stream now by pasting this statement into the ksqlDB window you opened at the beginning of this step. After you’ve created the stream, quit the ksqlDB CLI for now by typing exit.

Produce events to the input topic

Now let’s produce some records for the PURCHASE_STREAM stream

docker exec -i broker /usr/bin/kafka-console-producer --bootstrap-server broker:9092 --topic customer_purchases

After starting the console producer it will wait for your input. To send all send all the stock transactions click on the clipboard icon on the right, then paste the following into the terminal and press enter:

{"id": "1", "previous_purchase": 8000.54, "current_purchase": 5004.89,"txn_ts": "2020-12-04 02:35:43", "first_name": "Art","last_name": "Vandeley"}
{"id": "2", "previous_purchase": 500.33, "current_purchase": 1000.89,"txn_ts": "2020-12-04 02:35:44", "first_name": "Nick","last_name": "Fury"}
{"id": "3", "previous_purchase": 333.18, "current_purchase": 804.89,"txn_ts": "2020-12-04 02:35:45", "first_name": "Natasha","last_name": "Romanov"}
{"id": "4", "previous_purchase": 72848.11, "current_purchase": 60040.89,"txn_ts": "2020-12-04 02:35:46", "first_name": "Wanda","last_name": "Maximoff"}

After you’ve sent the records above, you can close the console producer with Ctrl-C.

Run the streaming report interactively with the ksqldb-cli

To begin developing interactively, open up the ksqlDB CLI:

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

Set ksqlDB to process data from the beginning of each Kafka topic.

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

Then let’s adjust the column width so we can easily see the results of the query

SET CLI COLUMN-WIDTH 20

Now we write a query to concatenate multiple columns. To achieve this, we will use the - operator to calculate the difference between two columns.

SELECT FIRST_NAME,
       LAST_NAME,
       CURRENT_PURCHASE - PREVIOUS_PURCHASE as PURCHASE_DIFF (1)
FROM PURCHASE_STREAM
EMIT CHANGES
LIMIT 4;

1	Using the `-` operator to calculate the difference between two columns.

The - operator expects numerical values. So if have columns where the numbers are stored as VARCHAR you’ll have to use a CAST operation to convert them to a numerical type, otherwise you’ll get an error in your query.

This query should produce the following output:

+--------------------+--------------------+--------------------+
|FIRST_NAME          |LAST_NAME           |PURCHASE_DIFF       |
+--------------------+--------------------+--------------------+
|Art                 |Vandeley            |-2995.6499999999996 |
|Nick                |Fury                |500.56              |
|Natasha             |Romanov             |471.71              |
|Wanda               |Maximoff            |-12807.220000000001 |
Limit Reached
Query terminated

Now that the reporting query works, let’s update it to create a continuous query for your reporting scenario:

CREATE STREAM PURCHASE_HISTORY_STREAM AS
  SELECT FIRST_NAME,
       LAST_NAME,
       CURRENT_PURCHASE - PREVIOUS_PURCHASE as PURCHASE_DIFF
FROM PURCHASE_STREAM;

We’re done with the ksqlDB CLI for now so go ahead and type exit to quit.

Write your statements to a file

Now that you have a series of statements that’s doing the right thing, the last step is to put them into a file so that they can be used outside the CLI session. Create a file at src/statements.sql with the following content:

CREATE STREAM PURCHASE_STREAM (
                ID VARCHAR,
                      PREVIOUS_PURCHASE DOUBLE,
                      CURRENT_PURCHASE DOUBLE,
                      TXN_TS VARCHAR,
                      FIRST_NAME VARCHAR,
                      LAST_NAME  VARCHAR)

 WITH (KAFKA_TOPIC='customer_purchases',
       VALUE_FORMAT='JSON',
       PARTITIONS=1);


CREATE STREAM PURCHASE_HISTORY_STREAM AS
  SELECT FIRST_NAME,
       LAST_NAME,
       CURRENT_PURCHASE - PREVIOUS_PURCHASE as PURCHASE_DIFF
FROM PURCHASE_STREAM;

Test it

Create the test data

Create a file at test/input.json with the inputs for testing:

{
  "inputs": [
    {
      "topic": "customer_purchases",
      "value": {
         "id": "1",
         "previous_purchase": 8000.54,
         "current_purchase": 5004.89,
         "txn_ts": "2020-12-04 02:35:43",
         "first_name": "Art",
         "last_name": "Vandeley"
      }
    },
    {
      "topic": "customer_purchases",
      "value": {
          "id": "2",
          "previous_purchase": 500.33,
          "current_purchase": 1000.89,
          "txn_ts": "2020-12-04 02:35:44",
          "first_name": "Nick",
          "last_name": "Fury"
      }
    },
    {
      "topic": "customer_purchases",
      "value": {
          "id": "3",
          "previous_purchase": 333.18,
          "current_purchase": 804.89,
          "txn_ts": "2020-12-04 02:35:45",
          "first_name": "Natasha",
          "last_name": "Romanov"
      }
    },
    {
      "topic": "customer_purchases",
      "value": {
         "id": "4",
         "previous_purchase": 72848.11,
        "current_purchase": 60040.89,
         "txn_ts": "2020-12-04 02:35:46",
         "first_name": "Wanda",
         "last_name": "Maximoff"
      }
    }
  ]
}

Create a file at test/output.json with the expected outputs:

{
  "outputs": [
    {
      "topic": "PURCHASE_HISTORY_STREAM",
      "value": {
          "FIRST_NAME" : "Art",
          "LAST_NAME" : "Vandeley",
          "PURCHASE_DIFF" : -2995.6499999999996
      }
    },
    {
      "topic": "PURCHASE_HISTORY_STREAM",
      "value": {
          "FIRST_NAME" : "Nick",
          "LAST_NAME" : "Fury",
          "PURCHASE_DIFF" : 500.56
      }
    },
    {
      "topic": "PURCHASE_HISTORY_STREAM",
      "value": {
          "FIRST_NAME" : "Natasha",
          "LAST_NAME" : "Romanov",
          "PURCHASE_DIFF" : 471.71
      }
    },
    {
      "topic": "PURCHASE_HISTORY_STREAM",
      "value": {
         "FIRST_NAME" : "Wanda",
         "LAST_NAME" : "Maximoff",
         "PURCHASE_DIFF" : -12807.220000000001
      }
    }
  ]
}

Invoke the tests

Invoke the tests using the ksqlDB test runner and the statements file that you created earlier:

docker exec ksqldb-cli ksql-test-runner -i /opt/app/test/input.json -s /opt/app/src/statements.sql -o /opt/app/test/output.json

Which should pass:

	 >>> Test passed!

Deploy on Confluent Cloud

Run your app with Confluent Cloud

Instead of running a local Kafka cluster, you may use Confluent Cloud, a fully managed Apache Kafka service.

Sign up for Confluent Cloud, a fully managed Apache Kafka service.
After you log in to Confluent Cloud Console, click Environments in the lefthand navigation, click on Add cloud environment, and name the environment learn-kafka. Using a new environment keeps your learning resources separate from your other Confluent Cloud resources.
From the Billing & payment section in the menu, apply the promo code CC100KTS to receive an additional $100 free usage on Confluent Cloud (details).
Click on LEARN and follow the instructions to launch a Kafka cluster and enable Schema Registry.

Next, from the Confluent Cloud Console, click on Clients to get the cluster-specific configurations, e.g., Kafka cluster bootstrap servers and credentials, Confluent Cloud Schema Registry and credentials, etc., and set the appropriate parameters in your client application.

Now you’re all set to run your streaming application locally, backed by a Kafka cluster fully managed by Confluent Cloud.