describing and storing data in a dbms

Describing and storing data in a DBMS


• RDBMS stands for Relational Database Management System.

• RDBMS is the basis for SQL, and for all modern database systems like MS SQL Server, IBM DB2, Oracle, MySQL, and Microsoft Access.

• Table• The data in RDBMS is stored in database objects

called tables. • The table is a collection of related data entries and it

consists of columns and rows.• Following is the example of a CUSTOMERS table:


• CUSTOMERS table


• Field• Every table is broken up into smaller entities

called fields. • The fields in the CUSTOMERS table consist of ID,

NAME, AGE, ADDRESS and SALARY.• A field is a column in a table.


• Record• A record, also called a row of data.• Individual entry that exists in a table. • For example there are 7 records in the above

CUSTOMERS table. • Following is a single row of data or record in the

CUSTOMERS table:

• A record is a horizontal entity in a table.


• column• A column is a vertical entity in a table.• For example, a column in the CUSTOMERS table

is ADDRESS, which represents location description and would consist of the following:


• NULL• A NULL value in a table is a value in a field that

appears to be blank, • which means a field with a NULL value is a field

with no value.• NULL value is different than a zero value or a

field that contains spaces. • A field with a NULL value is one that has been

left blank during record creation.


• Levels of Abstraction in a DBMS• Data Abstraction• For efficient retrieval of complex data, database

need a good data structures to represent data . • Developers hide the complexity from users

through several levels of abstraction, to simplify users’ interactions with the system:

• Physical Level : • Logical Level :• View Level :


• Levels of Abstraction in a DBMS• Data Abstraction• Physical Level : ( The lowest level ) • Logical Level : (The next-higher level )• View Level : (The highest level)


• Data Abstraction• Physical Level : The lowest level of abstraction

describes how the data are actually stored. • The physical level describes complex low-level

data structures in detail.


• Data Abstraction• Logical (Conceptual )Level : The next-higher level of

abstraction describes what data are stored in the database.

• And what relationships exist among those data. • The logical level describes the entire database in terms of

simple structures. • The user of the logical level does not need to be aware of

this complexity.• Database administrators, who must decide what

information to keep in the database, use the logical level of abstraction.


• Data Abstraction• Conceptual Schema• The conceptual schema (logical schema)

describes the stored data in terms of the data model of the DBMS.

• In a relational DBMS, the conceptual schema describes all relations that are stored in the database.

• Eg• Students(sid: int, name: char(20), age: int)


• Data Abstraction• View (External)Level : The highest level of

abstraction describes only part of the entire database.

• Many users of the database system do not need all this information; instead, they need to access only a part of the database.

• The view level of abstraction exists to simplify users interaction with the system.

• The system may provide many views for the same database.


• Data Abstraction• External Schema• Allow data access to be customized at the level

of individual users or groups of users.• Each external schema consists of a collection of

one or more views and relations from the conceptual schema.


• Data Independence

• Physical data independence• Logical data independence


• Data Independence• Physical data independence• The capacity to change the internal schema

without change the conceptual (or external) schema

• Internal schema may change to improve the performance

• (e.g., creating additional access structure)


• Data Independence• Logical data independence• The capacity to change the conceptual schema

without change external schema or application progms

• ex: Employee (E#, Name, Address, Salary)• A view including only E# and Name is not

affected by changes in any other attributes.


• QUERIES IN A DBMS• In order to obtain information from a database a

DBMS provides a specialized language, called the query language

• Relational calculus is a formal query language based on mathematical logic.

• SQL


• TRANSACTION MANAGEMENT• A transaction is anyone execution of a user

program in a DBMS. (Executing the same program several times will generate several transactions.)

• Partial transactions are not allowed, and the effect of a group of transactions is equivalent to some serial execution of all transactions.

• Transactions provide concurrency control• Transactions support crash recovery


• TRANSACTION MANAGEMENT• Four important properties (ACID):• Atomic: Database ensures that all actions are

carried out, or none• Consistency: Users ensure that transactions

leave the data in a consistent state• Isolation: Users to not have to worry about

concurrently executing transactions• Durability:Completed transactions persist after a

crash even if the database has not been updated


• TRANSACTION MANAGEMENT• Concurrent Execution of Transactions• DBMS is to schedule concurrent accesses to data

so that each user can safely ignore the fact that others are accessing the data concurrently.


• TRANSACTION MANAGEMENT• Concurrent Execution of Transactions• A locking protocol is a set of rules to be followed by each

concurrent transaction to ensure that, the transaction are in serial order.

• A lock is a mechanism used to control access to database objects.

• Two kinds of locks are commonly supported by a DBMS: • Shared locks :-for two different transactions at the same

time.• Exclusive lock :- for ensures that no other transactions

hold any lock on this object.


• STRUCTURE OF A DBMS• The DBMS accepts SQL commands and returns

the answers. • SQL commands can be embedded in host-

language application programs, e.g., Java or PHP programs.

• When a user issues a query, the query is passed to a query optimizer.

• It produces efficient execution plan about the stored data.


• STRUCTURE OF A DBMS• An execution plan is a blueprint for evaluating a

query, usually represented as a tree of relational operators

• Which bring the needed data to the main memory


• STRUCTURE OF A DBMS• The lowest layer of the DBMS software deals with

management of space ondisk, where the data is stored.

• Higher layers allocate, deal locate, read, and write pages through this layer, called the disk space manager.

• The DBMS maintain a log file, information about user, changes to the database…

• DBMS controls the concurrency control and crash recovery in the transactions


• STRUCTURE OF A DBMS• Transactions request and release locks according

to a suitable locking protocol• The lock manager, which keeps track of requests

for locks and grants• The recovery manager, which is responsible for

maintaining a log and restoring the system to a consistent state after a crash.

People who work with Databases

• The people who work with databases include

• database users, • system analysts, • application programmers, and • database administrator (DBA).


• Database users are those who interact with the database in order to query and update the database, and generate reports.

• Database users are further classified into

• Naive users:• Sophisticated users:• Specialized users:

People who work with Databases• Naive users: The users who maintain the database by invoking

some already written application programs. • For example, super market s/w user.

• Sophisticated users: The users, who are interact DBMS without writing any application programs.

• For example writing SQL

• Specialized users: The users who write specialized database programs,

• Specialized users write applications such as computer-aided design systems, knowledge-base, expert systems that store data having complex data types.


• System analysts determine the requirements of the database users (especially naive users) to create a solution for their business need, and focus on non-technical and technical aspects.

• The non-technical aspects involve defining system requirements, interaction between business users and technical staff, etc.

• Technical aspects involve developing the specification for user interface (application programs).


• Application programmers • are the computer professionals• who implement the specifications given by the

system analysts, • and develop application programs. • For the native users


• Database administrator (DBA) is a person who has central control over both data and application programs.

• Some of the responsibilities of DBA are• Design of the Logical and Physical Schemas:• The overall structure of the database is known

as database schema.• Data base schema is maintained by DDL.

http://my.safaribooksonline.com/9788131731925/gloss01


• Security and Authorization:• DBA is responsible for monitoring the security of

the database system. • It involves adding and removing users, auditing,

and checking for security problems.• New software installation: It is the responsibility

of the DBA to install new DBMS software, application software, and other related software.

• After installation, the DBA must test the new software.

People who work with Databases• Data analysis: DBA is responsible for analyzing the data

stored in the database, and studying its performance and efficiency in order to effectively use indexes, parallel query execution, etc.

• Routine maintenance checks: The DBA is responsible for taking the database backup periodically in order to recover from any hardware or software failure (if occurs). Other routine maintenance checks that are carried out by the DBA are checking data storage and ensuring the availability of free disk space for normal operations, upgrading disk space as and when required, etc.


• Data Availability and Recovery from Failures:

describing and storing data in a dbms

Documents

stored data

tablethe data

data model

lowest level logical

good data structures

logical level of abstraction

single row of data

higher level view level