Because they point to Web methods, Web Services proxy tables are of type procedure and are subject to all restrictions for this type:
Commands – You cannot issue a delete, update, or insert command against a table of type procedure, nor can you issue a create index, truncate table, or alter table command.
Joins – A Web Services proxy table can only be joined with one other table, and that table cannot be another Web Services proxy table.
Queries – Column names beginning with underscore ('_') are used to specify input parameters. These columns are referred to as parameter columns and must be in the where clause of a select statement.
For a complete description of the restrictions for procedure tables, see the Component Integration Services User’s Guide.
