A structure can be selected from, to obtain one of its fields (which in its turn may be a row, a structure or a simple value).
(315,040) (000,030)selection (000,010)(1,0)030060primary (1,0)030010. (1,0)030060field-name (1,0)030
The primary must yield a structure, one of whose fields has the field-name as name. The type of the selection is the type of the field yielded. As is the case for the subscription, the selection inherits the access attribute of its primary.