Update custom-detection-rules.md

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Louie Mayor 2020-08-11 16:06:58 -07:00
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@ -33,6 +33,10 @@ Custom detection rules built from [Advanced hunting](advanced-hunting-overview.m
In Microsoft Defender Security Center, go to **Advanced hunting** and select an existing query or create a new query. When using an new query, run the query to identify errors and understand possible results.
>[!IMPORTANT]
>To prevent the service from returning too many alerts, each rule is limited to generating only 100 alerts whenever it runs. Before creating a rule, tweak your query to avoid alerting for normal, day-to-day activity.
#### Required columns in the query results
To use a query for a custom detection rule, the query must return the `Timestamp`, `DeviceId`, and `ReportId` columns in the results. Simple queries, such as those that don't use the `project` or `summarize` operator to customize or aggregate results, typically return these common columns.