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Chapter 9: Object-Relational Databases
n Nested Relations
n Complex Types and Object Orientation
n Querying with Complex Types
n Creation of Complex Values and Objects
n Comparison of Object-Oriented and Object-Relational Databases
Object-Relational Data Models
n Extend the relational data model by including object orientation and constructs to deal with added data types.
n Allow attributes of tuples to have complex types, including non-atomic values such as nested relations.
n Preserve relational foundations, in particular the declarative access to data, while extending modeling power.
n Upward compatibility with existing relational languages.
Nested Relations
n Motivation:
Ø Permit non-atomic domains (atomic º indivisible)
Ø Example of non-atomic domain: set of integers,or set of tuples
Ø Allows more intuitive modeling for applications with complex data
n Intuitive definition:
Ø allow relations whenever we allow atomic (scalar) values — relations within relations
Ø Retains mathematical foundation of relational model
Ø Violates first normal form.
Example of a Nested Relation
n Example: library information system
n Each book has
Ø title,
Ø a set of authors,
Ø Publisher, and
Ø a set of keywords
n Non-1NF relation books
1NF Version of Nested Relation
n 1NF version of books
flat-books
4NF Decomposition of Nested Relation
n Remove awkwardness of flat-books by assuming that the following multivalued dependencies hold:
Ø title author
Ø title keyword
Ø title pub-name, pub-branch
n Decompose flat-doc into 4NF using the schemas:
Ø (title, author)
Ø (title, keyword)
Ø (title, pub-name, pub-branch)
4NF Decomposition of flat–books
Problems with 4NF Schema
n 4NF design requires users to include joins in their queries.
n 1NF relational view flat-books defined by join of 4NF relations:
Ø eliminates the need for users to perform joins,
Ø but loses the one-to-one correspondence between tuples and documents.
Ø And has a large amount of redundancy
n Nested relations representation is much more natural here.
Complex Types and SQL:1999
n Extensions to SQL to support complex types include:
Ø Collection and large object types
ê Nested relations are an example of collection types
Ø Structured types
ê Nested record structures like composite attributes
Ø Inheritance
Ø Object orientation
ê Including object identifiers and references
n Our description is mainly based on the SQL:1999 standard
Ø Not fully implemented in any database system currently
Ø But some features are present in each of the major commercial database systems
ê Read the manual of your database system to see what it supports
Ø We present some features that are not in SQL:1999
ê These are noted explicitly
Collection Types
n Set type (not in SQL:1999)
create table books (
…..
keyword-set setof(varchar(20))
……
)
n Sets are an instance of collection types. Other instances include
Ø Arrays (are supported in SQL:1999)
ê E.g. author-array varchar(20) array[10]
ê Can access elements of array in usual fashion:
– E.g. author-array[1]
Ø Multisets (not supported in SQL:1999)
ê I.e., unordered collections, where an element may occur multiple times
Ø Nested relations are sets of tuples
ê SQL:1999 supports arrays of tuples
Large Object Types
n Large object types
Ø clob: Character large objects
book-review clob(10KB)
Ø blob: binary large objects
image blob(10MB)
movie blob (2GB)
n JDBC/ODBC provide special methods to access large objects in small pieces
Ø Similar to accessing operating system files
Ø Application retrieves a locator for the large object and then manipulates the large object from the host language
Structured and Collection Types
n Structured types can be declared and used in SQL
create type Publisher as
(name varchar(20),
branch varchar(20))
create type Book as
(title varchar(20),
author-array varchar(20) array [10],
pub-date date,
publisher Publisher,
keyword-set setof(varchar(20)))
Ø Note: setof declaration of keyword-set is not supported by SQL:1999
Ø Using an array to store authors lets us record the order of the authors
n Structured types can be used to create tables
create table books of Book
Ø Similar to the nested relation books, but with array of authors
instead of set
n Structured types allow composite attributes of E-R diagrams to be represented directly.
n Unnamed row types can also be used in SQL:1999 to define composite attributes
Ø E.g. we can omit the declaration of type Publisher and instead use the following in declaring the type Book
publisher row (name varchar(20),
branch varchar(20))
n Similarly, collection types allow multivalued attributes of E-R diagrams to be represented directly.
n We can create tables without creating an intermediate type
Ø For example, the table books could also be defined as follows:
create table books
(title varchar(20),
author-array varchar(20) array[10],
pub-date date,
publisher Publisher
keyword-list setof(varchar(20)))
n Methods can be part of the type definition of a structured type:
create type Employee as (
name varchar(20),
salary integer)
method giveraise (percent integer)
n We create the method body separately
create method giveraise (percent integer) for Employee
begin
set self.salary = self.salary + (self.salary * percent) / 100;
end
Creation of Values of Complex Types
n Values of structured types are created using constructor functions
Ø E.g. Publisher(‘McGraw-Hill’, ‘New York’)
Ø Note: a value is not an object
n SQL:1999 constructor functions
Ø E.g.
create function Publisher (n varchar(20), b varchar(20))
returns Publisher
begin
set name=n;
set branch=b;
end
Ø Every structured type has a default constructor with no arguments, others can be defined as required
n Values of row type can be constructed by listing values in parantheses
Ø E.g. given row type row (name varchar(20),
branch varchar(20))
Ø We can assign (`McGraw-Hill’,`New York’) as a value of above type
Creation of Values of Complex Types
n Array construction
array [‘Silberschatz’,`Korth’,`Sudarshan’]
n Set value attributes (not supported in SQL:1999)
Ø set( v1, v2, …, vn)
n To create a tuple of the books relation
(‘Compilers’, array[`Smith’,`Jones’],
Publisher(`McGraw-Hill’,`New York’), set(`parsing’,`analysis’))
n To insert the preceding tuple into the relation books
insert into books
values
(`Compilers’, array[`Smith’,`Jones’],
Publisher(‘McGraw Hill’,`New York’ ),
set(`parsing’,`analysis’))
Inheritance
n Suppose that we have the following type definition for people:
create type Person
(name varchar(20),
address varchar(20))
n Using inheritance to define the student and teacher types
create type Student
under Person
(degree varchar(20),
department varchar(20))
create type Teacher
under Person
(salary integer,
department varchar(20))
n Subtypes can redefine methods by using overriding method in place of method in the method declaration
Multiple Inheritance
n SQL:1999 does not support multiple inheritance
n If our type system supports multiple inheritance, we can define a type for teaching assistant as follows:
create type Teaching Assistant
under Student, Teacher
n To avoid a conflict between the two occurrences of department we can rename them
create type Teaching Assistant
under
Student with (department as student-dept),
Teacher with (department as teacher-dept)
Table Inheritance
n Table inheritance allows an object to have multiple types by allowing an entity to exist in more than one table at once.
n E.g. people table: create table people of Person
n We can then define the students and teachers tables as subtables of people
create table students of Student
under people
create table teachers of Teacher
under people
n Each tuple in a subtable (e.g. students and teachers) is implicitly present in its supertables (e.g. people)
n Multiple inheritance is possible with tables, just as it is possible with types.
create table teaching-assistants of Teaching Assistant
under students, teachers
Ø Multiple inheritance not supported in SQL:1999
Table Inheritance: Roles
n Table inheritance is useful for modeling roles
n permits a value to have multiple types, without having a
most-specific type (unlike type inheritance).
Ø e.g., an object can be in the students and teachers subtables simultaneously, without having to be in a subtable student-teachers that is under both students and teachers
Ø object can gain/lose roles: corresponds to inserting/deleting object from a subtable
n NOTE: SQL:1999 requires values to have a most specific type
Ø so above discussion is not applicable to SQL:1999
Table Inheritance: Consistency Requirements
n Consistency requirements on subtables and supertables.
Ø Each tuple of the supertable (e.g. people) can correspond to at most one tuple in each of the subtables (e.g. students and teachers)
Ø Additional constraint in SQL:1999:
All tuples corresponding to each other (that is, with the same values for inherited attributes) must be derived from one tuple (inserted into one table).
ê That is, each entity must have a most specific type
ê We cannot have a tuple in people corresponding to a tuple each in students and teachers
Table Inheritance: Storage Alternatives
n Storage alternatives
H Store only local attributes and the primary key of the supertable in subtable
ê Inherited attributes derived by means of a join with the supertable
H Each table stores all inherited and locally defined attributes
ê Supertables implicitly contain (inherited attributes of) all tuples in their subtables
ê Access to all attributes of a tuple is faster: no join required
ê If entities must have most specific type, tuple is stored only in one table, where it was created
H Otherwise, there could be redundancy
Reference Types
n Object-oriented languages provide the ability to create and refer to objects.
n In SQL:1999
Ø References are to tuples, and
Ø References must be scoped,
ê I.e., can only point to tuples in one specified table
n We will study how to define references first, and later see how to use references
Reference Declaration in SQL:1999
n E.g. define a type Department with a field name and a field head which is a reference to the type Person, with table people as scope
create type Department(
name varchar(20),
head ref(Person) scope people)
n We can then create a table departments as follows
create table departments of Department
n We can omit the declaration scope people from the type declaration and instead make an addition to the create table statement:
create table departments of Department
(head with options scope people)
Initializing Reference Typed Values
n In Oracle, to create a tuple with a reference value, we can first create the tuple with a null reference and then set the reference separately by using the function ref(p) applied to a tuple variable
n E.g. to create a department with name CS and head being the person named John, we use
insert into departments
values (`CS’, null)
update departments
set head = (select ref(p)
from people as p
where name=`John’)
where name = `CS’
n SQL:1999 does not support the ref() function, and instead requires a special attribute to be declared to store the object identifier
n The self-referential attribute is declared by adding a ref is clause to the create table statement:
create table people of Person
ref is oid system generated
Ø Here, oid is an attribute name, not a keyword.
n To get the reference to a tuple, the subquery shown earlier would use
select p.oid
instead of select ref(p)
User Generated Identifiers
n SQL:1999 allows object identifiers to be user-generated
Ø The type of the object-identifier must be specified as part of the type definition of the referenced table, and
Ø The table definition must specify that the reference is user generated
Ø E.g.
create type Person
(name varchar(20)
address varchar(20))
ref using varchar(20)
create table people of Person
ref is oid user generated
n When creating a tuple, we must provide a unique value for the identifier (assumed to be the first attribute):
insert into people values
(‘01284567’, ‘John’, `23 Coyote Run’)
n We can then use the identifier value when inserting a tuple into departments
Ø Avoids need for a separate query to retrieve the identifier:
E.g. insert into departments
values(`CS’, `02184567’)
n It is even possible to use an existing primary key value as the identifier, by including the ref from clause, and declaring the reference to be derived
create type Person
(name varchar(20) primary key,
address varchar(20))
ref from(name)
create table people of Person
ref is oid derived
n When inserting a tuple for departments, we can then use
insert into departments
values(`CS’,`John’)
Path Expressions
n Find the names and addresses of the heads of all departments:
select head –>name, head –>address
from departments
n An expression such as “head–>name” is called a path expression
n Path expressions help avoid explicit joins
Ø If department head were not a reference, a join of departments with people would be required to get at the address
Ø Makes expressing the query much easier for the user
Querying with Structured Types
n Find the title and the name of the publisher of each book.
select title, publisher.name
from books
Note the use of the dot notation to access fields of the composite attribute (structured type) publisher
Collection-Value Attributes
n Collection-valued attributes can be treated much like relations, using the keyword unnest
Ø The books relation has array-valued attribute author-array and set-valued attribute keyword-set
n To find all books that have the word “database” as one of their keywords,
select title
from books
where ‘database’ in (unnest(keyword-set))
Ø Note: Above syntax is valid in SQL:1999, but the only collection type supported by SQL:1999 is the array type
n To get a relation containing pairs of the form “title, author-name” for each book and each author of the book
select B.title, A
from books as B, unnest (B.author-array) as A
n We can access individual elements of an array by using indices
Ø E.g. 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’
Unnesting
n The transformation of a nested relation into a form with fewer (or no) relation-valued attributes us called unnesting.
n E.g.
select title, A as author, publisher.name as pub_name,
publisher.branch as pub_branch, K as keyword
from books as B, unnest(B.author-array) as A, unnest (B.keyword-list) as K
Nesting
n Nesting is the opposite of unnesting, creating a collection-valued attribute
n NOTE: SQL:1999 does not support nesting
n Nesting can be done in a manner similar to aggregation, but using the function set() in place of an aggregation operation, to create a set
n To nest the flat-books relation on the attribute keyword:
select title, author, Publisher(pub_name, pub_branch) as publisher,
set(keyword) as keyword-list
from flat-books
groupby title, author, publisher
n To nest on both authors and keywords:
select title, set(author) as author-list,
Publisher(pub_name, pub_branch) as publisher,
set(keyword) as keyword-list
from flat-books
groupby title, publisher
n Another approach to creating nested relations is to use subqueries in the select clause.
select title,
( select author
from flat-books as M
where M.title=O.title) as author-set,
Publisher(pub-name, pub-branch) as publisher,
(select keyword
from flat-books as N
where N.title = O.title) as keyword-set
from flat-books as O
n Can use orderby clause in nested query to get an ordered collection
Ø Can thus create arrays, unlike earlier approach
Functions and Procedures
n SQL:1999 supports functions and procedures
Ø Functions/procedures can be written in SQL itself, or in an external programming language
Ø Functions are particularly useful with specialized data types such as images and geometric objects
ê E.g. functions to check if polygons overlap, or to compare images for similarity
Ø Some databases support table-valued functions, which can return a relation as a result
n SQL:1999 also supports a rich set of imperative constructs, including
Ø Loops, if-then-else, assignment
n Many databases have proprietary procedural extensions to SQL that differ from SQL:1999
SQL Functions
n Define a function that, given a book title, returns the count of the number of authors (on the 4NF schema with relations books4 and authors).
create function author-count(name varchar(20))
returns integer
begin
declare a-count integer;
select count(author) into a-count
from authors
where authors.title=name
return a=count;
end
n Find the titles of all books that have more than one author.
select name
from books4
where author-count(title)> 1
SQL Methods
n Methods can be viewed as functions associated with structured types
Ø They have an implicit first parameter called self which is set to the structured-type value on which the method is invoked
Ø The method code can refer to attributes of the structured-type value using the self variable
ê E.g. self.a
n The author-count function could instead be written as procedure:
create procedure author-count-proc (in title varchar(20),
out a-count integer)
begin
select count(author) into a-count
from authors
where authors.title = title
end
n Procedures can be invoked either from an SQL procedure or from embedded SQL, using the call statement.
Ø E.g. from an SQL procedure
declare a-count integer;
call author-count-proc(`Database systems Concepts’, a-count);
n SQL:1999 allows more than one function/procedure of the same name (called name overloading), as long as the number of
arguments differ, or at least the types of the arguments differ
External Language Functions/Procedures
n SQL:1999 permits the use of functions and procedures written in other languages such as C or C++
n Declaring external language procedures and functions
create procedure author-count-proc(in title varchar(20),
out count integer)
language C
external name’ /usr/avi/bin/author-count-proc’
create function author-count(title varchar(20))
returns integer
language C
external name ‘/usr/avi/bin/author-count’
n Benefits of external language functions/procedures:
Ø more efficient for many operations, and more expressive power
n Drawbacks
Ø Code to implement function may need to be loaded into database system and executed in the database system’s address space
ê risk of accidental corruption of database structures
ê security risk, allowing users access to unauthorized data
Ø There are alternatives, which give good security at the cost of potentially worse performance
Ø Direct execution in the database system’s space is used when efficiency is more important than security
Security with External Language Routines
n To deal with security problems
Ø Use sandbox techniques
ê that is use a safe language like Java, which cannot be used to access/damage other parts of the database code
Ø Or, run external language functions/procedures in a separate process, with no access to the database process’ memory
ê Parameters and results communicated via inter-process communication
n Both have performance overheads
n Many database systems support both above approaches as well as direct executing in database system address space
Procedural Constructs
n SQL:1999 supports a rich variety of procedural constructs
n Compound statement
Ø is of the form begin … end,
Ø may contain multiple SQL statements between begin and end.
Ø Local variables can be declared within a compound statements
n While and repeat statements
declare n integer default 0;
while n < 10 do
set n = n+1
end while
repeat
set n = n – 1
until n = 0
end repeat
n For loop
Ø Permits iteration over all results of a query
Ø E.g. find total of all balances at the Perryridge branch
declare n integer default 0;
for r as
select balance from account
where branch-name = ‘Perryridge’
do
set n = n + r.balance
end for
n Conditional statements (if-then-else)
E.g. To find sum of balances for each of three categories of accounts (with balance <1000, >=1000 and <5000, >= 5000)
if r.balance < 1000
then set l = l + r.balance
elseif r.balance < 5000
then set m = m + r.balance
else set h = h + r.balance
end if
n SQL:1999 also supports a case statement similar to C case statement
n Signaling of exception conditions, and declaring handlers for exceptions
declare out_of_stock condition
declare exit handler for out_of_stock
begin
…
.. signal out-of-stock
end
Ø The handler here is exit -- causes enclosing begin..end to be exited
Ø Other actions possible on exception
Comparison of O-O and O-R Databases
n Summary of strengths of various database systems:
n Relational systems
Ø simple data types, powerful query languages, high protection.
n Persistent-programming-language-based OODBs
Ø complex data types, integration with programming language, high performance.
n Object-relational systems
Ø complex data types, powerful query languages, high protection.
n Note: Many real systems blur these boundaries
Ø E.g. persistent programming language built as a wrapper on a relational database offers first two benefits, but may have poor performance.
Finding all employees of a manager
n Procedure to find all employees who work directly or indirectly for mgr
n Relation manager(empname, mgrname)specifies who directly works for whom
n Result is stored in empl(name)
create procedure findEmp(in mgr char(10))
begin
create temporary table newemp(name char(10));
create temporary table temp(name char(10));
insert into newemp -- store all direct employees of mgr in newemp
select empname
from manager
where mgrname = mgr
repeat
insert into empl -- add all new employees found to empl
select name
from newemp;
insert into temp -- find all employees of people already found
(select manager.empname
from newemp, manager
where newemp.empname = manager.mgrname;
)
except ( -- but remove those who were found earlier
select empname
from empl
);
delete from newemp; -- replace contents of newemp by contents of temp
insert into newemp
select *
from temp;
delete from temp;
until not exists(select* from newemp) -- stop when no new employees are found
end repeat;
end
End of Chapter
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