Applications that allow end users to enter or edit data often need to report errors to their users at the column level. The standard server message mechanism, however, makes column-level information available only within the text of the server message. Extended error data provides a means for applications to conveniently access column-level information.
For example, imagine an application that allows end users to enter and edit data in the titleauthor table in the pubs2 database. titleauthor uses a key composed of two columns, au_id and title_id. Any attempt to enter a row with an au_id and title_id that match an existing row will cause a “duplicate key” message to be sent to the application.
On receiving this message, the application needs to identify the problem column or columns to the end user, so that the user can correct them. This information is not available in the duplicate key message, except in the message text. The information is available, however, as extended error data.
