Trying to map a CDT to a view in my database. XSD attached. When I attempt to cr

Try using backticks when defining names with underscores:
@Column(name="`a_id`",columnDefinition="int",nullable=false) @Id

@johns222, Appian is trying to add primary key to your view (but Appian doesn't know that and it assumed, it's a table).
One workaround to this is to create your view with primary key from original table as well, and mark it as primary key when you create the CDT.
If you have no control over the view definition, you'll have to define composite primary key by modifying the XSD directly and use @Id in the annotation of multiple columns.

To the best of my knowledge, Appian does not support composite primary keys. One, and only one, of the fields in your CDT needs to be marked as the primary key. Because this is a view, it should not matter what field you pick since the contents of the view is determined by a query on the DB side. That said, @rawichp236 makes a good point that it is a good idea to include a primary key from one of your tables in your view to use as the primary key for your CDT.

The attached CDT has the @id defined. I guess the issue could be the _ as Jorge said try backticks `

Jorge's solution worked. Thank you!