sql database -english

8/9/2019 SQL Database -English

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In grammar, a modifier is an optional element in phrase structure or clause structure.[1] A modifier is so called because it is said to modify (change the meaning of) another element in the structure, on which it is dependent. Typically the modifier can be removed without affecting the grammar of the sentence.For example, in the English sentence This is a red ball, the adjective red is a modifier, modifying the noun ball. Removal of the modifier would leave This is a ball, which is grammatically correct and equivalent in structure to the original sentence.

qualifier (the word qualify may be used in the same way as modify in this context) an adjunct is an optional, or structurally dispensable, part of a sentence, clause, or phrase that, if removed or discarded, will not otherwise affect the remainder of the sentence. Example: In the sentence "John helped Bill in Central Park ", the phrase in Central Park is an adjunct. [1] DBMS

A DATABASE-MANAGEMENT SYSTEM (DBMS) is a collection of interrelated data and aset of p

rograms to access those data.

Primary goal - STORE and RETRIEVE database information that is both convenient and efficient.

DBMS- designed to manage large bodies of information- Management of data involves 1)defining structures for st

orage of information

2)providing mechanisms for the manipulation of information- ensure the safety of the information stored,- despite system crashes or attempts at unauthorized acces

s

Database System Applications - Banking,Airlines,universities,Credit card transactions,Telecommunication

Sales,Finance,Manufacturing,Human resources-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- FILE-PROCESSING SYSTEM -supported by a conventional operating system.

-stores permanent records in various files, and it needs different application programs to extract records from

and add records to, the appropr

iate files.-Before (DBMSs) came along, orga

nizations usually stored information in such systems.

DISADVANTAGES (file processing system):

·Data redundancy and inconsistency - information may be duplicated in several places (files)



- e.g. address & telephone number of a particular customer may appear in a file

that consists of savings-account records and in a file

that consists of checking-account records

- This redundancy leads to higher storage and access cost

- may lead data inconsistency that is, the various copies of the same data may no longer agree

- changed customer address may be reflected in savings-account records but not elsewhere in the system.

·Difficulty in accessing data -e.g. bank officers needs to find out the names of all customers who live withina particular postal-code area.

there is no application program on hand to meet it

·Data isolation -data are scattered in various files, and files may be in different formats,

writing new application programs to retrieve the appropriate data is difficult.

·Atomicity(happen anythingentirely or not at all)problems

- e.g. program to transfer $50 from account A to account B.If a system failure occurs during the execution of the program,

it is possible that the $50 was removed from account A but was not credited to account B,resulting in an inconsistent database state

·Concurrent-access anomalies-

·Security problems

-

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View of Data -major purpose of a database system is to provide userswith an abstract view of the dataData Abstraction -developers hide the complexity from users through several levels of abstraction, to simplify users' interactions with the system

· Physical level- lowest level ofabstraction describes how the data are actually stored. describes complex low-le

vel data structures in detail.· Logical level. The next-higher l

evel of abstraction describes what data are stored in the database, and what relationships exist among those data.

may involve complex physical-level 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.



· View level. The highest level of abstraction describes only part of the entire database. Even though the logical level uses simpler structures, complexity remains because of the

variety of information stored in a large 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 their interaction with the system. The system may provide many views for the same database.

sql database -english

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