# ClickHouse



ClickHouse is an open-source, column-oriented database designed for real-time analytics. It can process very large amounts of data quickly and efficiently, making it a great option for business intelligence and high-performance reporting.

## What you can do with this integration

The integration for ClickHouse lets you:

* **Read users from a ClickHouse database** and unify them as user profiles inside Krenalis.
* **Write unified users back into ClickHouse** and keep the target table synchronized over time.

You can use it both to **collect user data** and to **activate user profiles**.

<ul class="cards" data-columns="2">
  <li>
    <a href="/docs/ingest-users/databases?database=clickhouse">
      <figure><img src="%3cp%3e/docs/integrations/images/clickhouse.svg%3c/p%3e%0a" alt="ClickHouse"></figure>
      <div>Ingest users</div>
      <p>Read and sync user data from ClickHouse into your workspace.</p>
    </a>
  </li>
  <li>
    <a href="/docs/activate-profiles/databases?database=clickhouse">
      <figure><img src="%3cp%3e/docs/integrations/images/clickhouse.svg%3c/p%3e%0a" alt="ClickHouse"></figure>
      <div>Activate users</div>
      <p>Write unified profiles to ClickHouse and keep the data updated.</p>
    </a>
  </li>
</ul>

## Do incremental imports in query

If the incremental import is enabled, you must use the `updated_at` placeholder in the query, as shown in the following example:

```
SELECT first_name, last_name, phone_number
FROM customers
WHERE updated_at >= ${updated_at}
ORDER BY updated_at
```

The column used in the `WHERE` statement must be the same column selected as the update time column in the pipeline, and the query must return the rows ordered by this column in ascending order. For example, if the update time column is a datetime column and the update time is `2025-01-30 16:12:25.837`, the executed query would be:

```
SELECT first_name, last_name, phone_number
FROM customers
WHERE updated_at >= '2025-01-30 16:12:25.837'
ORDER BY updated_at
```

If incremental import is not selected, `${updated_at}` will be `NULL`. To make the query work whether or not incremental import is enabled, you can write it as follows:

```
SELECT first_name, last_name, phone_number
FROM customers
WHERE updated_at >= ${updated_at} OR ${updated_at} IS NULL 
ORDER BY updated_at
```