1 Theory, Practice & Methodology of Relational Database Design and Programming Copyright © Ellis Cohen Introduction to Objects & Databases These slides are licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 2.5 License. For more information on how you may use them, please see

2 © Ellis Cohen Topics Object Mapping and ODL Object Query Language Object Relational Mapping Object-Oriented Databases (OODB's) Summary

3 © Ellis Cohen Object Mapping and ODL

4 © Ellis Cohen Drivers for OO & DB Integration Persistence Object Storage –Need for a way to easily save and restore computation state of programs built via OOPL's + other RDB benefits OO Client-Side Programming for RDB's –Treat rows of a table (plus asssociated data in other tables) like an object for client program access & modification OO Server-Side Programming for RDB's –Brings benefits of OO approach to RDB's

5 © Ellis Cohen Object & Relational Features Relational Model Persistence Storage Optimization Indexing Queries (SQL) Query Optimization Constraints & Triggers Transactions Backup & Recovery Object Model Encapsulation Object Types Attributes Methods Inheritance Polymorphism Object Identity References Navigation Collections Versioning

6 © Ellis Cohen Rows as Objects/Entities empno: 7654 ename : MARTIN sal : 1250 comm : 1400 an Employee Object It can be useful to think of each row as an object or entity (i.e. an instance of an entity class) and the table as a collection of these objects The columns of the table correspond to the instance variables for each object It can be useful to think of each row as an object or entity (i.e. an instance of an entity class) and the table as a collection of these objects The columns of the table correspond to the instance variables for each object empno ename sal comm Emps 7499ALLEN MARTIN BLAKE KING TURNER STERN1500

7 © Ellis Cohen Object Mapping Relational Mapping Mapping an ER model to a Relational Model Object Mapping Mapping an ER model to an Object Model Dept works for deptno dname empno ename job address Employee manages [manager] [worker]

8 © Ellis Cohen Object Model Object Classes & Instances –Correspond to entity classes and instances Collections Set (no duplicates) Bag (duplicates) Lists (ordered, duplicates) Array (list w efficient indexing) References (Pointers)

9 © Ellis Cohen Object Definition Language (ODL) class Employee { attribute int empno; attribute string ename; attribute string job; attribute Struct[street,city,state,zip] address; relationship Dept dept inverse Dept::empls; } class Dept { attribute int deptno; attribute string dname; relationship Set empls inverse Employee::dept;

10 © Ellis Cohen ODL Relationships dept1 dept2 dept3 emp1 emp3 emp5 emp4 emp2 empls dept

11 © Ellis Cohen ODL Exercise Represent the manages relationship in ODL

12 © Ellis Cohen Representing Manages class Employee { attribute int empno; attribute string ename; attribute string job; attribute Struct[street,city,state,zip] address; relationship Dept dept inverse Dept::empls; relationship Employee mgr inverse managees; relationship Set managees inverse mgr; } class Dept { attribute int deptno; attribute string dname; relationship Set empls inverse Employee::dept;

13 © Ellis Cohen Referencing / Navigation Given an employee e e.dept - department of e e.dept.deptno – the # of e's dept e.dept.dname – the name of e's dept Given a department d d.empls – the set of employees who work in dept d

14 © Ellis Cohen OQL (Object Query Language)

15 © Ellis Cohen OQL (Object Query Language) Given a department d d.empls the collection of employees who work in dept d SELECT e.empno FROM e IN d.empls the employee numbers of the employees who work in department d SELECT e.empno, e.ename FROM e IN d.empls the employee numbers & names of the employees who work in department d OQL SELECT iterates through the objects in a collection

16 © Ellis Cohen OQL SELECT Returns Collections SELECT e.empno FROM e IN d.emps the employee numbers of the employees who work in department d Bag SELECT e.empno, e.ename FROM e IN d.emps the employee numbers & names of the employees who work in department d Bag SELECT e FROM e IN d.emps WHERE e.job = 'CLERK' the clerks who work in department d Set

17 © Ellis Cohen Extents and Keys How do iterate through all the employees? We can associate an extent with a class, which corresponds to the set of instances in that class. We can associate a key with an extent

18 © Ellis Cohen ODL with Extents class Employee (extent emps, key empno) { attribute int empno; attribute string ename; attribute string job; attribute Struct[street,city,state,zip] address; relationship Dept dept inverse Dept::empls; relationship Employee mgr inverse managees; relationship Set managees inverse mgr; } class Dept (exptent depts, key deptno) { attribute int deptno; attribute string dname; relationship Set empls inverse Employee::dept;

19 © Ellis Cohen Collections and Relationships depts emps dept1 dept2 dept3 emp1 emp3 emp5 emp4 emp2 empls dept

20 © Ellis Cohen Queries involving Extents SELECT e.empno FROM e IN emps WHERE e.ename = 'SMITH' SELECT e.empno, e.ename FROM e IN emps WHERE e.job = 'CLERK'

21 © Ellis Cohen Joins & Navigation SELECT e.ename, d.dname FROM e IN emps, d IN depts WHERE e.dept = d –An ordinary expensive join SELECT e.ename, e.dept.dname FROM e IN emps WHERE e.dept IS NOT NULL –Lists every employee & their dept –Replaces joins by navigation SELECT e.ename, d.dname FROM d IN depts, e IN d.empls –Lists the employees in each department –Uses correlated navigation

22 © Ellis Cohen Object-Relational Mapping

23 © Ellis Cohen Object Relational Mapping The idea of OR Mapping is to treat a RDB in terms of its object model. In particular –the DB is modelled in ODL –queries are written in ODL –ODL queries are automatically mapped to SQL

24 © Ellis Cohen Mapping OQL to SQL SELECT e.ename, e.dept.dname FROM emps e in OQL is mapped to the SQL: SELECT e.ename, (SELECT d.dname FROM depts d WHERE e.deptno = d.deptno) FROM emps e A good optimizer will treat this as equivalent to SELECT ename, dname FROM emps NATURAL JOIN depts

25 © Ellis Cohen Returning Collections of Objects SELECT e FROM e IN emps WHERE e.job = 'ANALYST' This query returns a collection of objects (i.e. the employees who are analysts) to the client Using an ordinary OO programming language (e.g. Java/C++), it is possible to update these objects directly, and then arrange for the changes to be automatically reflected in the corresponding database objects on commit.

26 © Ellis Cohen Embedded OQL Programming Imagine a PL with embedded OQL a (possible) server-side language for an OODB clerkEmps Set := SELECT e FROM e IN emps WHERE job = 'CLERK'; FOR e IN clerkEmps LOOP IF e.dept.dname = 'ACCOUNTING' THEN e.sal := e.sal * 1.1; END IF; END LOOP; COMMIT; /* Once the transaction has been committed, the changed objects are persisted to the database */

27 © Ellis Cohen Middle-Tier Caching Most OO Models for databases allow code manipulating DB objects to be in the middle-tier as well Objects are brought over to the middle-tier as the result of a query, while updated objects are sent back to the database upon commit. Often uses an optimistic concurrency control

28 © Ellis Cohen Current Approaches JDO –Java standard for database object modelling embedded in Java –Uses JDOQL, a variant of OQL, with very limited capabilities –Many commercial implementation, including KODO UML –UML's Model Driven Architecture assumes that people will do design at the UML level, and have it mapped automatically onto real implementation –OCL is an assertion language that is based on UML's object model.

29 © Ellis Cohen Object-Oriented Databases (OODB's)

30 © Ellis Cohen Types of Databases Hierarchical DB Network DB XML DB OO/OR DB Relational DB (RDB) Deductive DB MultiDimensional DB

31 © Ellis Cohen Timeline for Database Systems Before 1960 transition from punched card and tape 1960s, from file management to databases IMS from IBM, Hierarchical Data Model IMS DB/DC, Network Model and communication SABRE, multi-user access with network 1970s, CODASYL and Relational Model Codd (IBM) Relational Model Chen introduced Entity Relationship Model Query languages developed (SQL) 1980s, Client/Server RDBs, Oracle, DB2 PC databases, DBase, Paradox, etc. SQL standard for definition and manipulation 1990s, web-based information delivery Object DB's for Object Persistence Multidimensional DB's for Data Warehousing Deductive Databases for Data Mining XML DB's for Semi-structured Data

32 © Ellis Cohen OODB's In an OODB Objects exist independently, not just as rows in a table An object may have a reference to another object (allowing navigation) Instead of a table, there are collections, which contain references to objects

33 © Ellis Cohen Object Identity Primary Key Identifies row in an RDB Can be changed or reused Not usually globally unique Reference via foreign key, may not necessarily always refer to same row OID (Object ID) Uniquely identifies object, independent of all of its values (e.g. my axe) Can't be changed; also can't be reused (unlike ROWIDs) Sometimes is globally unique (e.g. also includes IP address & DB id ) A reference effectively holds an OID, and always refers/points to the same object

34 © Ellis Cohen ROWIDs vs OIDs ROWIDs uniquely identify rows as long as they exist If a row is deleted, the ROWID can be reused. An OID (Object ID) is like a ROWID, but it is never reused OIDs are used to identify and refer to objects in OODBs An OID could be constructed from a ROWID plus a usage-number e.g. The OID AAAGDxAABAAAH9EAAD27 is the 27th use of the ROWID AAAGDxAABAAAH9EAAD Other UID techniques are widely used Why?

35 © Ellis Cohen Extents & Objects in ODB's 622AuditingCHICAGO… deptno dname loc … Objects from different extents/classes may all share the same block! 6291SMITH…AAAGDxAABA AAH9EAAD27 an Emp Objects are not in tables; each one is independent!

36 © Ellis Cohen Summary

37 © Ellis Cohen ORDB's and OODB In an ORDB A row may have a direct references to another row (allowing navigation) A row can be used as an object A table acts like a collection of objects In an OODB Objects exist independently, not just as rows in a table An object may have a reference to another object (allowing navigation) Instead of a table, there are collections, which contain references to objects

38 © Ellis Cohen OR Databases In an ORDB A table may be defined as a collection of objects, where every row in the table corresponds to an object A row object is uniquely identified by its OID An object may have a reference to another row object (allowing navigation) SELECT e.ename, e.dept.loc FROM emps e The ORDB iterates through the table emps For each employee object e in emps, it extracts the ename and dept attributes The dept attribute is a department object reference, so the ORDB navigates to the corresponding department row object (in the depts table) and extracts the loc attribute.

39 © Ellis Cohen OO Databases In an OODB Objects exist independently, not just as rows in a table An object is uniquely identified by its OID An object may have a reference to another object (allowing navigation) Instead of tables, there are collections, which contain references to objects SELECT e.ename, e.dept.loc FROM e IN emps The OODB iterates through the collection emps For each employee e referenced in emps, it navigates to that employee object, and extracts the ename and dept attributes The dept attribute is a department object reference, so the OODB navigates to the department and extracts the loc attribute.

40 © Ellis Cohen OR Mapping Layer When using an OR Mapping Layer An ordinary RDB table may be treated as a collection of objects, where every row in the table corresponds to an object A row object is uniquely identified by the combination of its table name and its primary key. Foreign keys act like references to the object in the foreign table with that primary key. SELECT e.ename, e.dept.loc FROM e IN emps emps and depts are ordinary tables, with primary keys empno and deptno, respectively. The emps table contains deptno as well, which is a foreign key referencing the depts table. The OR mapping layer maps the original OO query to a corresponding pure SQL query.