In an external table schema, the columns that can typically be included are related to metadata that assist in identifying and managing the data in the external location. VALUE is usually not a column that can be included in an external table schema as it's too generic. METADATA$ROW_ID is essential for uniquely identifying each row. METADATA$FILENAME helps track the source file of the data. METADATA$FILE_ROW_NUMBER is useful for managing row-specific information from a file.