Friday, 29 May 2015

Databases and Database Users

Databases are used in every part of day-to-day life.  Examples of common database use include: depositing or withdrawing money from a bank, making a travel reservation, accessing a library catalog, buying something from the internet via a web page, etc.

These are examples of traditional database applications, where data is stored either in textual or numeric format.  Less traditional database applications that are starting to become more popular include multimedia databases, which store pictures, video clips, and sounds, Geographic Information Systems (GIS) that store maps, satellite images and weather data.

Introduction and Definitions

 Database – a collection of related data.
 Data – facts that can be recorded and have implicit meaning

For example, you may record the names, telephone numbers and addresses of everyone you know, either using an address book, or storing on a hard drive and accessing it using applications such as MS Access and Excel.  Here you are using a database, a collection of related data with an implicit meaning.

Implicit properties of a database include:
  • A database represents some aspect of the real world, sometimes called the mini world, or Universe of Discourse.  Changes to the mini world are reflected in the database.
  • A database is a logically coherent collection of data with some inherent meaning.  A random assortment of data cannot be correctly referred to as a database.
  • A database is designed, built and populated with data for a specific purpose.  It has an intended group of users and some preconceived applications in which these users are interested.

So to summarize, a database has some source from which data is taken, a degree of interaction with the events in the real world, and an audience that is actively interested in the contents of a database.

The size and complexity of a database can vary.  The Bilkent library database may include hundreds of thousands of items, whereas a small company may maintain a database for only 30 employees.

A database is not necessarily computerized.  It can be generated and maintained manually, or it may be computerized.

 Database Terminology

Any firm that gathers extensive amounts of information to be retrieved at some point in the future depends on a file system (generally a database system) that allows for the access of specific information in a timely and cost effective manor. This information is usually stored in files, records and fields.

A file is a set of related information stored together.  It is a collection of information relative to a number of related entities.  For example, a bank needs to collect various types of information about customers, employees, savings accounts, demand accounts, etc.  Each one of these is a ‘subpackage’ of information, stored as a file.  They may have 4 files, and Employee File, a Customer File, a Savings Account file and a Demand Account file. 

A record is a collection of information about an entity.  If we look at the example from above, the Customer File, a record is a collection of information about a customer entity.  The record contains information required by the bank about each customer, and stored for each customer.  For example, information contained in a customer record may include at least first_name, last_name, id_number, birth_date.  Each of these pieces of information are taken together to form a customer record.  Because each record in the file contain the same number and types of fields, the records have the same type.

A field is a piece of information about an entity, contained in a record.  In the above example, the 4 pieces of information, first_name, last_name, id_number and birth_date are all fields.

Therefore we can use the following definitions, a record is a collection of related fields, and a file is a collection of related records, of the same type.

Insertion, Deletion, Update

The information being stored in files needs to be maintained.  This maintenance requires three operations, Insertion, Deletion and Update.

Keys

In order to satisfy many requests (such as “Find the Balance of account 123-456”), one must first retrieve the corresponding record from the file, and examine the contents of the balance field.  In order to retrieve the record, it must first be accessed.  To access the correct customer record, the first step is to locate the target file which contains the information to be retrieved.  In this case it may be the Accounts File.  Then from the Accounts file, the target record must be located among all the records in the file, using some access mechanism.  The target record will be one in which the account number field value is equal to 123-456.

In most cases there is a field in a record that identifies that particular record uniquely.  This field is referred to as the primary key of the record.   In some cases, there may not be a single field that uniquely identifies a record and serve as the primary key.  In these cases, a combination of two or more fields can serve as the primary key.  This is called a composite key.

Let’s assume there is a field for Customer Name.  In this case, because it is likely that two customers will have the same name, therefore duplicate values, Customer Name can’t be used as a primary key.  This is referred to as a secondary key.  A secondary key is a field which identifies a record, but that does not uniquely identify a record. 

In some cases there may be more than one field that uniquely identifies a record.  For example, if there was a field for customer ID number, it should be unique for all records.  In this case,  the fields that could also serve as the primary key are called candidate keys.  The database designer must choose one candidate key to serve as the primary key.

Database Management System (DBMS)

Definition: a collection of programs that enables users to create and maintain a database. 
A general-purpose software system that makes it possible to define, construct, manipulate and share databases among various users and applications.

§  Defining: specifying the data types, structures and constraints for the data to be stored in the database.
§  Constructing: the process of storing the data itself on some storage medium that is controlled by the DBMS
§  Manipulating: includes such functions as querying the database to retrieve specific data, updating the database to reflect changes in the mini world, and generating reports from the data.
§  Sharing: allows multiple users and programs to access the database concurrently.
Other functions include protecting the database and maintaining it over a long period of time. 
§  Protection: includes system protection against software/hardware malfunction and security protection against unauthorized/malicious access.
§  Maintaining: the DBMS should allow the database to evolve as requirements change over time.

Databases can be implemented by general purpose DMBSs, or special purpose custom software can be written to create and maintain a database.
  
Database System: database and DBMS software together.

 
Database System

Example: University Database

Used to maintain information concerning students, courses and grades in a university environment. 

The database has five files, each which stores data records of the same type.

§  The Student file stores data of each student
§  The Course file stores data on  each course,
§  The Section file stores data on each section of a course,
§  The Grade_Report stores information on the grades students receive in the various sections they have completed,
§  The Prerequisite file stores the prerequisite of each course.

DEFINE: To define the database, we must specify the structure of the records in the file by specifying the different types of data elemets to be stored in each record (elements of each file)

CONSTRUCT: To construct the database, we store data to represent each student, course, section, grade report and prerequisite as a record in the appropriate file.  The records in various files may be related.  Records for Smith in the Student file is related to two records in the Grade_Report file that specify Smith’s grades in tow sections.  Each record in the Prerequisite file relates two course records, on representing the course and the other representing the prerequisite.

Most medium to large databases include many types of records, and have many relationships among the records.

MANIPULATE: To manipulate involves querying and updating.  For example, to retrieve the list of all courses and Grades of Smith is a query.  An example of an update is to change the class of Smith to 2.  These informal queries and updates must first be translated to the query language used by the DBMS to be processes.


Characteristics of the Database Approach

A number of characteristics distinguish the database approach from the traditional approach of programming with files.  In traditional file processing, each user defines and implements the files needed for a specific software application as part of programming the application.

For example, one user, the grade reporting office, may keep a file on students and their grades.  Programs to print a transcript and to enter new grades into the file are implemented as part of the application.  A second user, the accounting office, may keep track of students fees and their payments.  Although both are interested in the data about students, each user maintains separate files and programs to manipulate the files because each requires some data not available from the other user’s files.

This redundancy in defining and storing data results in wasted storage space and in redundant efforts to maintain common, up-to-date data.

With the database approach, a single repository of data is maintained that is defined once and is then accesses by various users.  The main characteristics of the database approach versus file processing are as follows:

§  Self describing nature of the database system
§  Insulation between programs and data and data abstraction
§  Support of multiple views of the data
§  Sharing of data and multi-user transaction processing.

Self-Describing Nature of a Database System


§  A fundamental characteristic f the database approach is that the database system contains not only the database itself, but a complete definition or description of the database structure and constraints.
§  The definition is stored in the DBMS catalog which contains information such as the structure of each file, the type and storage format of each data item, and various constraints on the data.
§  The information stored in the catalog is called metadata, and it describes the structure of the primary database.
§  The catalog is used by the DBMS software and also by database users who need information about the database structure.
§  A general purpose database application is not written for a specific database application, so it must refer to the catalog to know the structure of the files in a specific database, such as the type and format of data it will access.
§  The DBMS software must work equally well with any number of database applications.
§  In traditional file processing, the data definition is part of the application programs themselves; hence they are constrained to work with only a specific database (i.e. a banking database, or a university database).  For example, an application written in C++ may have a strict or class declaration.
§  File processing software can access only specific databases; DBMS software can access diverse databases by extracting the database definitions from the catalog, and using these definitions.

Example: In the example database, the catalog will store the definitions of all files shown.  The database designer prior to creating the actual database specifies the definitions.  Whenever a request is made to access for example the name of a student record, the DBMS software refers to the catalog to determine the structure of the student file and the position and size of the name data item within a student record.  In traditional file processing, the size and file structure are coded within the application that accesses the data item.

Insulation between Programs and Data, and Data Abstraction

§  In traditional file processing, the structure of the data files is embedded in the application programs so any changes to the structure of a file may require changing all programs that access the file.

§  By contrast, DBMS access programs in most cases do not require such changes.  The structure of data files is stored in the catalog separately from the access programs.  This is called program-data independence.

§  For example, a file access program may be written so that it can only access student records with the exact structure specified by the starting position and length.  If we add another piece of data to the student record, such as BirthDate, the program must then be changed.

§  In a DBMS environment, we just need to change the description of a student record in the catalog to reflect the inclusion of the new data item BirthDate, and no programs need to be changed.  The next time a program refers to the catalog, the new structure of Student records will be accessed and used.

§  In some types of database systems, such as Object Oriented database systems users can define operations on data as part of the database.

§  An operation is specified in two parts.  The interface of an operation includes the operation name and the data types of its arguments.  The implementation of the operation is specified separately and can be changed without affecting the interface.

§  User application programs can operate on the data by invoking the operations through their names and arguments regardless of how the operations are implemented.  This can be called program-operation independence.

§  This is called data abstraction.  A DBMS provides users with the conceptual representation of data that does not include details of how the data is stored or how the operations are implemented.  A data model is a type of data abstraction that is used to provide the conceptual representation.

Example:  The internal implementation of a file may be defined by its record length (the number of characters in each record), and each data item may be specified by its starting byte within a record and its length in bytes.  But a typical database user is not concerned with the location of each data item within a record or its length. 

The DBMS hides details of the file storage organization from the user.

Support of Multiple Views of the Data

§  A database typically has many users, each of whom may require a different perspective or view of the database.
§  A view may be a subset of the database, or it may contain virtual data that is derived from the database files, but not explicitly stored.  (Give examples of these types of data)
§  Users generally do not need to know if the data is stored or derived.

Sharing of Data and Multi-User Transaction Processing

§  A multi-user DBMS must allow multiple users to access the database at the same time.
§  This is important if data for multiple applications is to be integrated (brought together) and maintained in a single database.
§  The DBMS must include concurrency control software to ensure that several users trying to update the same data do so in a controlled manor.
§  For example, when several reservation clerks try to assign a seat on a flight, the DBMS should ensure that each seat can be accesses by only one clerk.
§  An important concept in database applications is a transaction.  It is an executing program or process that includes on or more database accesses, such as reading or updating of database records. 
§  Each transaction is supposed to execute a logically correct database access if executed in its entirety without interference from other transactions.
§  The isolation property ensures that each transaction appears to execute in isolation from other transactions, even though hundreds of transactions may be executing concurrently.

Database Users

Database administrators – DBA is responsible for authorizing access to the database, for coordinating and monitoring its use, and acquiring software and hardware resources as needed.

Database designers – identify data to be stored in the database and choosing appropriate structures to represent and store the data.  Most of these functions are done before the database is implemented and populated with the data.  It is the responsibility of the database designers to communicate with all prospective users to understand their requirements and come up with a design that meets these requirements.  Database designers interact with all potential users and develop views of the database that meet the data and processing requirements of these groups.  The final database must support the requirements of all user groups.

End Users
§  Casual End Users – occasionally access, may need different information each time.  Use query language to specify requests.
§  Naïve or parametric end users – main job is to query and update the database using standard queries and updates.  These canned transactions have been carefully programmed and tested. Examples?
§  Sophisticated end users – engineers, scientists, analysts who implement applications to meet their requirements.
§  Stand alone users – maintain personal databases using ready made packages.

System Analysts and Programmers
§  Determine the end user requirements (especially naïve and parametric end users) and develop specifications for canned transactions that meet the requirements.

§  Application programmers implement the specifications as programs, then test and debug the programs.
Propellerads