We can't compare external lists with standard lists where data is stored inside of SharePoint. Since the data is stored outside SharePoint, there are many Limitations in External Lists. Many of the Ribbon features will not be available for external Lists.
- Datasheet view is not available
- Export to Excel option is not available
- Workflows cannot be configured for External Lists
- Cannot create Information Management Policies (bar codes, auditing, retention or labels)
- No version or version history
- Inline Editing is not available
- No Ratings widget
- Can't create Visio Diagrams
- Open with Access/Open with Project is not supported
- REST access via ListData.svc to External Lists is not available (All WCF services like RSS feeds are not available)
- No Look ups - No Content Types
- No Context Menu (ECB) Send To operations.
- Events handlers are not supported.
- LINQ to SharePoint spmetal.exe does not support external lists
- No Item Level Permissions
- No item or field level validation or Formula supported
- Document templates are not supported.
- Alerts are not supported
- Attachments will not be available
- Additional data connections in InfoPath 2010 list forms not supported
- My favorite Utility: U2U CAML Query builder doesn't work!
- No Check In / Check Out options
- Drafts of items are not supported.
- No write support for BLOB
- Metadata based navigation is not supported
Tail: You cannot mix the SharePoint columns with External List columns since they are coming from external source.
Update: In SharePoint 2013, following improvements are made:
- Sorting & Filtering for External lists is improved
- Export to Excel option included
- Event Receiver on external lists is introduced.
- Supports OData
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