====== Deletion of Historical Data ======

This guide explains how to permanently remove incorrect or unwanted historical values from a datapoint — for example, a single erroneous spike caused by a sensor glitch that doesn't reflect the surrounding trend.

> **Warning:** Deleting historical data from the database is permanent and cannot be undone. Always double-check your selection before confirming, and make sure you understand the difference between removing data from the web display only versus permanently deleting it from the database (see Step 2 below).

===== Prerequisites =====

Deleting historical data requires elevated project permissions — this is not available to standard users by default.

  - In the **Mervis Portal**, go to your project's **Rights** settings.
  - Click **Show Detailed Permissions** to reveal two additional permission types: **Set DP History** and **Delete DP History**.
  - You must have the **Delete DP History** right assigned to your account in order to delete historical data.

===== Step 1: Identify the Data to Delete =====

  - Open the project in **Mervis SCADA** (web interface).
  - Navigate to the **Graph** view for the datapoint containing the data you want to remove.
  - Click **Show/Hide Series Points** to display the individual data points along the series — this makes it much easier to click and select the exact point(s) you intend to delete.
  - Carefully select only the data you want to remove. Since this action can permanently affect the database, confirm your selection matches your intent before continuing.

**Example use case:** A sensor briefly malfunctions and records a single unrealistic spike in an otherwise smooth temperature trend. Using **Show/Hide Series Points**, you can isolate and select just that one erroneous point without affecting the surrounding valid data.

===== Step 2: Delete the Selected Data =====

  - With your data selected, trigger the delete action. A **Confirm Action** dialog will appear.
  - This first dialog offers two options:
    * **Yes** — Deletes the selected data **from the web GUI display only**. The underlying data in the database is not affected.
    * **No** — Does not cancel the operation. Instead, a **second Confirm Action dialog** appears.
  - If a second dialog appears, it will ask you to confirm **permanent deletion from the database**. Confirming this step **permanently and irreversibly removes the selected data** from the underlying database.

> **Note:** Because clicking "No" on the first dialog leads toward permanent deletion rather than cancelling, always read each confirmation dialog carefully before clicking anything.

===== Related Pages =====

  * **[[en:mervis-scada:90-how-to:30-graphs|Graphs]]** – General guide to visualizing and navigating historical data.
  * **[[en:mervis-scada:90-how-to:33-upload-historical-data|Upload Historical Data]]** – The reverse operation: importing historical data into a datapoint.