Relation-Valued Attributes

1. Our extended SQL allows an expression evaluating to a relation to appear anywhere that a relation name may appear. The ability to use subexpressions freely makes it possible to take advantage of the structure of nested relations.
2. The schema for a relation pdoc.

    
   		 create table pdoc
   
   		 		 (name MyString,
   
   		 		  author-list setof(ref(people)),
   
   		 		  date MyDate,
   
   		 		  keyword-list setof(MyString))
   
   

3. To find all documents having a keyword ``database'',

    
   		 select name
   
   		 from pdoc
   
   		 where ``database'' in keyword-list
   
   

4. To find pairs of the form ``doc-name, author-name'' for each document and each author of the document,

    
   		 select B.name, Y.name
   
   		 from pdoc as B, B.author-list as Y
   
   

5. Aggregate functions can be applied to any relation-valued expression.

    
   		 select name, count(author-list)
   		 from pdoc