Centralized

8/11/2019 Centralized

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Centralized Vs DistributedCentralized Systems:A modern, general-purpose computer system consists of one to a few

CPUs & a number of device controllers that are connected through a common bus that

provides access to shared memory. The CPUs have local cache memories that store local

copies of parts of the memory, to speed up access to data. Each device controller is in charge

of a specific type of device (for example, a disk drive, an audio device, or a video display). The

CPUs & the device controllers can execute concurrently, competing for memory access. Cache

memory reduces the contention for memory access, since it reduces the number of times

that the CPU needs to access shared memory.In a distributed database system, the database is stored on several computers. The

computers in a distributed system communicate with one another through various

communication media, such as high-speed networks or telephone lines. They do not share

main memory or disks. The computers in a distributed system may vary in size & function,

ranging from workstations up to mainframe systems.

The main difference between centralized & distributed databases is that the distributed

databases are typically geographically separated, are separately administered, & have slower

interconnection. Also in distributed databases we differentiate

between local& globaltransactions. A local transactionis one that accesses data only from

sites where the transaction originated. A global transaction, on the other hand, is one that

either accesses data in a site different from the one at which the transaction was initiated, or

accessed data in several different sites.


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E R Diagram

Advantages

Conceptual simplicity

Visual representation

Effective communication

Integration with the relational database model

Disadvantages

Limited constraint representation

Limited relationship representation No representation of data manipulation

Loss of information


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Using a Subquery

to Solve a Problem

Who has a salary greater than Abels?

Which employees have salaries greater

than Abels salary?

Main query:

What is Abels salary?

Subquery:


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The subquery (inner query) executes once beforethe main query (outer query).

The result of the subquery is used by the mainquery.

SELECT select_list

FROM table

WHERE expr operator

(SELECT select_list

FROM table);

Subquery Syntax


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SELECT last_name, salary

FROM employees

WHERE salary >

(SELECT salary

FROM employeesWHERE last_name = 'Abel');

Using a Subquery

11000


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Guidelines for Using Subqueries

Enclose subqueries in parentheses.

Place subqueries on the right side of the

comparison condition.

The ORDER BYclause in the subquery is

not needed unless you are performing Top-

N analysis.

Use single-row operators with single-rowsub queries, and use multiple-row operators

with multiple-row sub queries.


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Types of Subqueries

Single-row subquery

Multiple-row subquery

Main query

Subqueryreturns

ST_CLERK

ST_CLERK

SA_MAN

Main query

Subquery

returns


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Single-Row Subqueries

Return only one row

Use single-row comparison operators

Operator Meaning

= Equal to

> Greater than

>= Greater than or equal to

< Less than


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SELECT last_name, job_id, salary

FROM employees

WHERE job_id =

(SELECT job_id

FROM employeesWHERE employee_id = 141)

AND salary >

(SELECT salary

FROM employees

WHERE employee_id = 143);

Executing Single-Row Subqueries

ST_CLERK

2600


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SELECT last_name, job_id, salary

FROM employees

WHERE salary =

(SELECT MIN(salary)

FROM employees);

Using Group Functions in a Subquery

2500


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Multiple-Row Subqueries

Return more than one row

Use multiple-row comparison operators

Operator Meaning

IN Equal to any member in the list

ANY Compare value to each value returned by the

subquery

ALL Compare value to every value returned by

the subquery


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SELECT employee_id, last_name, job_id, salary

FROM employees

WHERE salary < ANY

(SELECT salary

FROM employeesWHERE job_id = 'IT_PROG')

AND job_id 'IT_PROG';

Using the ANYOperator

in Multiple-Row Subqueries

9000, 6000, 4200


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SELECT employee_id, last_name, job_id, salary

FROM employees

WHERE salary < ALL

(SELECT salary

FROM employeesWHERE job_id = 'IT_PROG')

AND job_id 'IT_PROG';

Using the ALLOperator

in Multiple-Row Subqueries

9000, 6000, 4200


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SELECT emp.last_name

FROM employees emp

WHERE emp.employee_id NOT IN

(SELECT mgr.manager_id

FROM employees mgr);

no rows selected

Null Values in a Subquery

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