Database Architecture


1st there was two level architecture including schema & subschema …

but now a 3 level architecture

external level : the users view of db, it includes that part ofdb which s relevant to

 each user..

conceptual level : the community view of db, it includes all entities , their att &

relationships , constraints on data, semantic info abt data,  security & interity info.

internal level : the physical rep of db on comp, it inculudes storage spce, record

description,  record placement,  data compressin, data encryption..


Data Definition language: DDL used to define the db schema (data types, str,

constraints) e,g  SQL..

Data Manipulation language: DML s used to both read & update db(insert, updat,

del, retrieve) by data manipulation language (DML) r of two types

    a .  procedural lang : manipulate db record by record, allow users to state how t

oretrieve the data e,g c , C++ , java

    b.   non procedural lang : manipulates sets of records of db allow users to statewat

 data s needed e,g  SQL,QBE (query by examp)..

4GL : fourth generation language ::e,g SQL, QBE..

ENTITY :: s a distinct object (a person, place, thing , concept, event)

SEQUAL SERVER :: s a Ms sw whose name s SQL – SERVER