معابنة
cg.. a prcnut rcceIved nuy conmist of data itcms policy numntet and aoxunt. but nny
eist xeveral tines for oe poliey.
ach ocutenee ofadata item ₩ill have vpecife walue (for evampie tie wabus of premniun
wount' my he '¥91 25" or the vnlue of vehiele type' may he 'alon eurl; theaewlues will
be rpesentadl in a suitable code.
these logical data units cun be tunsferred fairly cusily fom the clerical cxample to onc
where programs. DBMS, and direet access storage replace the mamger, filing clerk. and
iling eabinebs. Pwugruns nay repue¥t mn wxeurente uf a SLET by providing eitner i pnmury
or u xcondlary key (the primary key of a member rcorl). A nutnber of
Syate aalyais wd desigN
Dr. Ai Alyaaree
Wasit Univer-iy- coupulet aeieace wod tT tueuy
RECORDS will hen he rerieved for the pmgram. whieh will manipulate the DATA
ITtMS unl DATA AGGREGATES wun therein
Ihysieal chtabase unit
input to and output from the cental ocessor in a computer t¥stem is accomnplished by a
wunmand whish ewues a secundar¥ slorage Jevice to be reud Imom oae indicator oا
noker, and the duta thus found uansfered to main mcmor¥y- In the case ot 1 mugneti
mape, the indiewnrs will be imter shek gaps: in the euse af magnetie dlisks or dlnums, the
irndicantors will be anlilress tarher.
All he dlata reconled berween thee indie-tar form a phy sieal neorl sometinea known a
a bloek or sestor. A PAGE may be ome or mare of these (eontiguous) physical rcotds.la
practice. i is unhkely that a page will he looger tuan a tck of a magnetic disk, and
ج
houl be poisible to perforn a multiple ptysical rxcord reud in a shont tinc to rctieve
puge. Where itis possible to vury the size of phyaieal recoeds in a computerwystem. positive
perturmunee xhvantges wil be pined by muking the page 2e exaetly ewwal to the
pyseal reunl se. A puge cun eontain u number of reeonls. and a reeurd can spun mre
nan one page. There may he different reenrd rypes to " puge heemre there is no retictinr
m menrpage rehtiomship
ARFA is simply a eolleeton of pages, nomally contigunuis. There is no need for an
as1 to be reticted to disk pack. e¥linder oF tuck bouindares. as an alca wil consist of 1
nultiple of pages. The specitication of areas provides a mcanns of physically scgmenting
s2eoondury ¥ornge: the puge is ¥enerally that pan of an irea whieh is rend into ntain orige
MeveFy PU or
put comman.
k is the job ot mh DEMS t mateh requsk tom progcanns forset or nocord oreurenees
and page ideniners (This is the COpASYI- appronch. othner appmaches are diftcrent
in delail.) The database upprouch enables the introduction of the concepts of VIRIUAL
ht and TRANSPARENT datu: VIRTUAL dat is the tern siven to dutu item types whose
walues are ealeuluted fmin other torl la items anl presenteu to a pgram only when
they ure resued by that pogrum. TRANSPARENT Jutdl is the
Sytem analyeas and deiyn
D.Ai Alydare
Wari Uetversty- cutpw₩er cienee -d T facahy
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