Defining the Views of the Global Schema¶

Once the base relations have been defined and their corresponding wrappers constructed, each relation of the global schema is defined through a query on the base relations in a manner similar to that used to define views on a conventional database.

It is important to highlight that when defining a view, in addition to the base relations, previously defined views can also be used.

Example: Take three base relations

A and B represent two electronic bookshops on the Internet. C represents a source in which users review books and the system allows the average review for a specific book to be searched for. Imagine that we want to obtain a relation of the global schema

R = {TITLE, AUTHOR, PRICE, AVERAGE_RELEVANCE)

that contains all the books of A and B, together with the average review according to C and the minimum value found for the PRICE attribute, amongst the occurrences of the book found in both sources. R can be defined in the two following steps:

1. Creating the view bookview like the union of A and B.

CREATE VIEW bookview AS
   SELECT * FROM A
   UNION
   SELECT * FROM B;

2. Creating the view R as the join of bookview and C by applying the GROUP BY operation to select the minimum price for each book.

CREATE VIEW R AS
   SELECT bookview.TITLE
      , bookview.AUTHOR
      , AVERAGE_RELEVANCE,
   MIN(PRICE) AS MINIMUM
   FROM
      bookview JOIN C
      ON bookview.TITLE = C.TITLE
      AND bookview.AUTHOR = C.AUTHOR
   GROUP BY bookview.TITLE, bookview.AUTHOR, AVERAGE_RELEVANCE;

As mentioned above, the base views can present limitations in their query capabilities, which are expressed through search methods. When creating views Virtual DataPort can automatically calculate its search methods from those of the base relations and from the statement used to define the view. This allows the system to know a priori if a specific query can be answered.