LEVELS OF DB 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
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..
DB LANGUAGES ::
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