An external table is a schema entity that references data stored outside the Azure Data Explorer database. Azure Data Explorer Web UI can create external tables by taking sample files from a storage container and creating schema based on these samples. You can then analyze and query data in external tables without ingestion into Azure Data Explorer. For information about different ways to create external tables, see create and alter external tables in Azure Storage or Azure Data Lake.
One of the most common scenarios for External table is with historian data (e.g. data that need to be stored due to legal requirements, log records for longer retention period, etc.) that need to be query rarely.
Please read create an external table document for detailed explanation, here are some highlighted points.
1. at the Source Page, In Link to source, enter the SAS URL of your source container. You can add up to 10 sources (You can remove the 10 limitation by using the create external table command at the Query page). The first source container will display files below the File filters. In a later step, you will use one of these files to generate the table schema.
2. At the Schema Page, in the right-hand side of the tab, you can preview your data. On the left-hand side, you can add partitions to your table definitions to access the source data more quickly and achieve better performance.
3. At the Summary Page, you can query this table using the query buttons or with external_table() function. For more information on how to query external tables, see Querying an external table.
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