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How to concatenate column values in ksqlDB

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How to concatenate column values in ksqlDB

In this tutorial, we'll show how to use the concatenation operator in ksqlDB to create a single value from multiple columns.

Setup

Assume that we have a stream named activity_stream that simulates stock purchases and serves as our example for concatenating column values.

CREATE STREAM activity_stream (
        num_shares INT,
        amount DOUBLE,
        txn_ts VARCHAR,
        first_name VARCHAR,
        last_name  VARCHAR,
        symbol VARCHAR)
    WITH (KAFKA_TOPIC='activity-stream',
          VALUE_FORMAT='JSON',
          PARTITIONS=1);

Concatenate column values

Now let's create a stream that concatenates several columns from activity_stream to create a summary of activity.

CREATE STREAM activity_summary AS
  SELECT first_name + ' ' + last_name +
       ' purchased ' +
       CAST(num_shares AS VARCHAR) +
       ' shares of ' +
       symbol AS summary
  FROM activity_stream
  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 activity_stream stream backed by Kafka running in Docker and populate it with test data.

CREATE STREAM activity_stream (
        num_shares INT,
        amount DOUBLE,
        txn_ts VARCHAR,
        first_name VARCHAR,
        last_name  VARCHAR,
        symbol VARCHAR)
    WITH (KAFKA_TOPIC='activity-stream',
          VALUE_FORMAT='JSON',
          PARTITIONS=1);

INSERT INTO activity_stream (num_shares, amount, txn_ts, first_name, last_name, symbol)
    VALUES (300, 5004.89, '2024-09-04 02:35:43', 'Tony', 'Stark', 'IMEP');
INSERT INTO activity_stream (num_shares, amount, txn_ts, first_name, last_name, symbol)
    VALUES (425, 1000.91, '2024-09-04 02:35:44', 'Nick', 'Fury', 'IMEP');
INSERT INTO activity_stream (num_shares, amount, txn_ts, first_name, last_name, symbol)
    VALUES (918, 8004.54, '2024-09-04 02:35:45', 'Natasha', 'Romanov', 'STRK');
INSERT INTO activity_stream (num_shares, amount, txn_ts, first_name, last_name, symbol)
    VALUES (100, 6088.22, '2024-09-04 02:35:46', 'Wanda', 'Maximoff', 'STRK');

Finally, run the concatenation query and then select from it. Note that we first tell ksqlDB to consume from the beginning of the stream.

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

CREATE STREAM activity_summary AS
  SELECT first_name + ' ' + last_name +
       ' purchased ' +
       CAST(num_shares AS VARCHAR) +
       ' shares of ' +
       symbol AS summary
  FROM activity_stream
  EMIT CHANGES;
SELECT *
FROM activity_summary
EMIT CHANGES;

The query output should look like this:

+-----------------------------------------------------------+
|SUMMARY                                                    |
+-----------------------------------------------------------+
|Tony Stark purchased 30000 shares of IMEP                  |
|Nick Fury purchased 425 shares of IMEP                     |
|Natasha Romanov purchased 918 shares of STRK               |
|Wanda Maximoff purchased 100 shares of STRK                |
+-----------------------------------------------------------+

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 activity_stream stream and populates it with test data.

CREATE STREAM activity_stream (
        num_shares INT,
        amount DOUBLE,
        txn_ts VARCHAR,
        first_name VARCHAR,
        last_name  VARCHAR,
        symbol VARCHAR)
    WITH (KAFKA_TOPIC='activity-stream',
          VALUE_FORMAT='JSON',
          PARTITIONS=1);

INSERT INTO activity_stream (num_shares, amount, txn_ts, first_name, last_name, symbol)
    VALUES (300, 5004.89, '2024-09-04 02:35:43', 'Tony', 'Stark', 'IMEP');
INSERT INTO activity_stream (num_shares, amount, txn_ts, first_name, last_name, symbol)
    VALUES (425, 1000.91, '2024-09-04 02:35:44', 'Nick', 'Fury', 'IMEP');
INSERT INTO activity_stream (num_shares, amount, txn_ts, first_name, last_name, symbol)
    VALUES (918, 8004.54, '2024-09-04 02:35:45', 'Natasha', 'Romanov', 'STRK');
INSERT INTO activity_stream (num_shares, amount, txn_ts, first_name, last_name, symbol)
    VALUES (100, 6088.22, '2024-09-04 02:35:46', 'Wanda', 'Maximoff', 'STRK');

Now paste the query to concatenate column values in the editor and click Run query:

CREATE STREAM activity_summary AS
  SELECT first_name + ' ' + last_name +
       ' purchased ' +
       CAST(num_shares AS VARCHAR) +
       ' shares of ' +
       symbol AS summary
  FROM activity_stream
  EMIT CHANGES;

Finally, select from the resulting stream:

SELECT *
FROM activity_summary
EMIT CHANGES;

The query output should look like this:

+-----------------------------------------------------------+
|SUMMARY                                                    |
+-----------------------------------------------------------+
|Tony Stark purchased 30000 shares of IMEP                  |
|Nick Fury purchased 425 shares of IMEP                     |
|Natasha Romanov purchased 918 shares of STRK               |
|Wanda Maximoff purchased 100 shares of STRK                |
+-----------------------------------------------------------+

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>