Note I have not tried this myself, but can you try the following and let me know if it helps:

1. Create the query, so SELECT name, date FROM errors;
2. Set up the alert. Set the condition to trigger the alert when the count of rows is greater than 1.
3. Create a separate query that formats the result as a single string.

   1. SELECT CONCAT_WS(', ', COLLECT_LIST(CONCAT(name, ' - ', date))) AS formatted_result
      FROM errors
      WHERE <your_condition>;

   2. Use this query in your alert to trigger the notification. The formatted_result should contain the concatenated rows.
4. Send notification to Slack. In the alert settings, you can use the formatted_result to include the actual rows in the message.