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Here, oid is an attribute name, not a keyword The subquery above would then use select poid instead of select ref(p) An alternative to system-generated identi ers is to allow users to generate identi ers The type of the self-referential attribute must be speci ed as part of the type de nition of the referenced table, and the table de nition must specify that the reference is user generated: create type Person (name varchar(20), address varchar(20)) ref using varchar(20) create table people of Person ref is oid user generated When inserting a tuple in people, we must provide a value for the identi er: insert into people values ( 01284567 , John , 23 Coyote Run ) No other tuple for people or its supertables or subtables can have the same identi er We can then use the identi er value when inserting a tuple into departments, without the need for a separate query to retrieve the identi er: insert into departments values ( CS , 01284567 ) It is even possible to use an existing primary key value as the identi er, by including the ref from clause in the type de nition: create type Person (name varchar(20) primary key, address varchar(20)) ref from(name) create table people of Person ref is oid derived Note that the table de nition must specify that the reference is derived, and must still specify a self-referential attribute name When inserting a tuple for departments, we can then use insert into departments values ( CS , John )
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Object-Relational Databases
95 Querying with Complex Types
In this section, we present extensions of the SQL query language to deal with complex types Let us start with a simple example: Find the title and the name of the publisher of each book This query carries out the task: select title, publishername from books Notice that the eld name of the composite attribute publisher is referred to by a dot notation
951 Path Expressions
References are dereferenced in SQL:1999 by the > symbol Consider the departments table de ned earlier We can use this query to nd the names and addresses of the heads of all departments: select head >name, head >address from departments An expression such as head >name is called a path expression Since head is a reference to a tuple in the people table, the attribute name in the preceding query is the name attribute of the tuple from the people table References can be used to hide join operations; in the preceding example, without the references, the head eld of department would be declared a foreign key of the table people To nd the name and address of the head of a department, we would require an explicit join of the relations departments and people The use of references simpli es the query considerably
952 Collection-Valued Attributes
We now consider how to handle collection-valued attributes Arrays are the only collection type supported by SQL:1999, but we use the same syntax for relation-valued attributes also An expression evaluating to a collection can appear anywhere that a relation name may appear, such as in a from clause, as the following paragraphs illustrate We use the table books which we de ned earlier If we want to nd all books that have the word database as one of their keywords, we can use this query: select title from books where database in (unnest(keyword-set)) Note that we have used unnest(keyword-set) in a position where SQL without nested relations would have required a select-from-where subexpression
Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
III Object Based Databases and XML
9 Object Relational Databases
The McGraw Hill Companies, 2001
Querying with Complex Types
If we know that a particular book has three authors, we could write: select author-array[1], author-array[2], author-array[3] from books where title = Database System Concepts Now, suppose that we want a relation containing pairs of the form title, authorname for each book and each author of the book We can use this query: select Btitle, Aname from books as B, unnest(Bauthor-array) as A Since the author-array attribute of books is a collection-valued eld, it can be used in a from clause, where a relation is expected
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