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Object-Relational Model

Object-Relational Model. Review: Data Models. Hierarchical Network ER (Pure) Relational (Pure) Object-oriented (ODMG) Object-relational (since SQL:1999) Semistructured (XML). Two Approaches to OO+DB. The OO community: pure OO The SQL community: bring the best of OO into RDB world.

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Object-Relational Model

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  1. Object-Relational Model

  2. Review: Data Models • Hierarchical • Network • ER • (Pure) Relational • (Pure) Object-oriented (ODMG) • Object-relational (since SQL:1999) • Semistructured (XML)

  3. Two Approaches to OO+DB • The OO community: pure OO • The SQL community: bring the best of OO into RDB world. • SQL:1999, SQL:2003 OODB ORDB OO RDB ODL/OQL SQL-99

  4. Review: OO Concepts • Type sytem • Atomic types • Type constructors: struct, collection, reference • Classes and Objects • OID • Methods • Class hierarchy • Inheritance

  5. From Relation to Object-Relation • Complex type system for attributes • Structure and set/bag • Esp.: bag of structs = relation • Nested relation • Methods • Tuple ID: tuples play the role of objects, and have OID’s. • Hidden, but can be made visible to users. • References: pointers to tuples

  6. Nested Relations • Type of an attribute can be a relation schema. • Example Students(sno,name,age,courses(cno,title,credit)) • Redundancy? CTX(course,teachers(name),texts(title)) • MVD?

  7. References • Type of an attribute can be a reference to a tuple with a given schema or a set of references to tuples with a given schema. • A(*R): A’s type is a reference to a tuple with schema R. • A({*R}): A’s type is a set of references to tuples with schema R. • Eliminate redundancy in nested-relations. • Example Course(cno,title,credit) Student(sno,name,age,courses({*Course}))

  8. OO vs OR • Objects vs. tuples • How is relationship represented? • Methods • Type systems • References and OID’s • Backwards compatibility

  9. ODL to OR • More direct than ODL to Relations • Methods can be translated now.

  10. User-Defined Types in SQL • UDT in SQL = Classes in ODL • UDT is used in two ways • A UDT can be the type of a relation; i.e., the type of its tuples. • Sometimes called a row type. • A UDT can be the type of an attribute in a relation.

  11. UDT Declaration • Renaming of an existing type: CREATE TYPE type-name AS primitive-type; • To enforce stronger type checking. • Class-like UDT: CREATE TYPE type-name AS (list of attribute decalarations) list of methods; • Don’t treat relationship as property, but represent it by a separate relation or through reference.

  12. Example CREATE TYPE HeightType AS INTEGER; CREATE TYPE WeightType AS INTEGER; CREATE TYPE AddressType AS ( street CHAR(50), city CHAR(20) ); CREATE TYPE Person AS ( name CHAR(30), address AddressType );

  13. Methods in UDT • Need both a declaration and a definition • Example CREATE TYPE AddressType AS ( street CHAR(50), city CHAR(20)) METHOD houseNum( ) RETURNS CHAR(10); CREATE METHOD houseNum( ) RETURNS CHAR(10) FOR AddressType BEGIN …PSM language … END;

  14. Creating a Table using a UDT • Syntax CREATE TABLE table-name OF UDT-name (list-of-elements); • Tuples must be thought of as objects. • List-of-elements: key, FK, Check constraints. • For the table, not for the UDT. • Example CREATE TABLE people OF Person (PRIMARY KEY name);

  15. Typed Table • A table that is declared to be based on some structured type is called a typed table. • its columns correspond in name and declared type to the attributes of the structured type. • Typed tables have one additional column, called the “self-referencing column” whose type is a reference type associated with the structured type of the table.

  16. Subtable vs. Supertable • If a typed table TB1 has an associated structured type TP1 that is a subtype of some other structured type TP2, then TB1 can be defined to be a “subtable” of a typed table TB2 whose associated type is TP2; TB2 is, in this case, a “supertable” of TB1.

  17. Reference Column • UDT tables may have a reference column that serves as the “identity” for its tuples, like OID. The table is then referenceable. • Primary key of the table • Generated and maintained unique by the DBMS • Declaration CREATE TABLE tablename OF typename ( REF IS attribute-name how-generated ); • SYSTEM GENERATED • DERIVED: use the PK

  18. References • For an attribute A to refer to tuples of a table R of UDT T with a reference column: A REF(T) SCOPE R • if no scope is specified, the reference can go to any relation of T

  19. Example CREATE TYPE Person AS ( ... ); CREATE TABLE people OF Person ( REF IS pID SYSTEM GENERATED, PRIMARY KEY (name)); CREATE TYPE Department AS ( name CHAR(30), head REF(Person) SCOPE people);

  20. Many-Many Relationships • Suppose we have declared: CREATE TYPE StudentType AS …; CREATE TABLE students OF StudentType; CREATE TYPE CourseType AS …; CREATE TABLE courses OF CourseType; Then we may represent the many-many relationship between students and courses by references: CREATE TABLE SC ( sref REF(StudentType) SCOPE students, cref REF(CourseType) SCOPE courses);

  21. Operations on OR Data • All SQL applies. • Some new operations

  22. Following References • If x is of type REF(T), then x refers to some tuple t of type T. • x->a is the value of attribute a in t. • DEREF(x) is the tuple t. • Example SELECT DEREF(sref) FROM SC WHERE sref->name = ‘James Bond’;

  23. Accessing Components of Tuples • Every UDT has an implicitly defined observer methodx( ) for each attribute x. • If t is variable of type T, then t.x( ) is the value of x in the tuple (object) denoted by t.

  24. Example: Observer Method • Example SELECT s.name() FROM students s WHERE s.sno() = ‘007’; • Tuple (object) variable is needed. • ( ) of observer method may be omitted in practice. • Not for other methods. Lu Chaojun, SJTU

  25. Generator and Mutator Methods • When a UDT T is defined, two other methods are defined automatically: • Generator method: T( ) returns an object of type T, with no values in its various components. • Mutator method: t.x(v) sets v to attribute x of tuple t.

  26. Ordering Relationships on UDT • To specify an ordering or comparison on UDT objects. 1. CREATE ORDERING FOR T EQUALS ONLY BY STATE; • Equality of corresponding components 2. CREATE ORDERING FOR T EQUALS ONLY BY RELATIVE WITH F; • Equality by test F(x1,x2) = 0. 3. CREATE ORDERING FOR T ORDERING FULL BY RELATIVE WITH F; • Ordering by testing F(x1,x2): <0 for <, =0 for =, >0 for >.

  27. Example CREATE ORDERING FOR AddressType ORDERING FULL BY RELATIVE WITH AddrLEG; CREATE FUNCTION AddrLEG( x1 AddressType, x2 AddressType ) RETURNS INTEGER IF x1.city() < x2.city() THEN RETURN(-1) ELSEIF x1.city() > x2.city() THEN RETURN(1) ELSEIF x1.street() < x2.street() THEN RETURN(-1) ELSEIF x1.street() = x2.street() THEN RETURN(0) ELSE RETURN(1) END IF;

  28. End

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