> For the complete documentation index, see [llms.txt](https://docs.akenza.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.akenza.io/device-management/rule-engine/logic-blocks/rule-logic.md).
# Comparison
Compare device data to a constant or to the last sample of a device.
The **Comparison** logic block compares values from device data samples. It needs no code and consists of two sections:
* **Data sources** define which values the block reads.
* **Conditions** define how those values are compared.
A comparison logic block
## Data sources
A data source selects a **topic** of the incoming data samples and the **data key** to read from them. Only samples with that topic trigger the block. Use `*` to match all topics.
If the data flow of the device has a [device type](/device-management/your-data-flow/device-type.md) with a schema, you can choose the data key from the schema. Otherwise, enter the topic and data key by hand.
A comparison needs at least one data source. Add more if the conditions need more values. The same device can be assigned to several data sources, one for each key you want to evaluate.
{% hint style="info" %}
To find the topic and data key of a sample, open the **Message Logs** on the [device detail view](/device-management/create-new-device/devices.md).
{% endhint %}
Topic and data keys in the message logs
## Conditions
A condition compares a data source, with a comparison operator, to either:
* a **constant number**, or
* the **latest sample** of another data source.
Click the blue text to switch between `is` and `is not`. Use the **plus** button to nest another comparison inside a condition.
Conditions are independent of each other. Each condition that evaluates to true triggers the actions linked to it.
{% hint style="info" %}
If a comparison is not enough, use a [Custom Logic](/device-management/rule-engine/logic-blocks/custom-logic.md) block.
{% endhint %}
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