Definition
Inthedatabase:Rawdata,alsoknownasuserdata,isdatathathasnotbeenprocessedorsimplified.Thesedatamayormaynotbeinmachine-readableform.Itconstitutesdatathatphysicallyexists.Generallythereareseveralbasicdatatypesinthedatabase.
Metadata
Metadataisthedatainthesystemdatabase,anditsmainbodyisthedatadictionary.Thedatadictionaryisacollectionofvariousdescriptioninformationandcontrolinformationinthedatabasesystem.Metadataisusedtodescribethecharacteristicsofotherdataandprovidebackgroundinformationandmeaningdata.Generally,metadataisalsousedtodescribethedatastructure,dataelements,internalrelationshipsandothercharacteristicsofthedata.Italsoallowsdatatobetracedtotheindividualwhogeneratedthedata.Thepurposeofitsuseistoidentifyresources;evaluateresources;trackchangesintheuseofresources;realizesimpleandefficientmanagementoflargeamountsofnetworkeddata;realizeeffectivediscovery,search,integratedorganizationofinformationresources,andeffectivemanagementofusedresources.
Basictype
Charactertype
char[(length)]fixed-lengthcharacterupto2000bytes
varchar2[(length)]Variable-lengthcharacterdatatype,upto4000bytes
NCHAR[(length)]Fixed-lengthcharacterdatatype,composedofcharactersfromtheNationalLanguageSupport(NLS)characterset,Upto2000bytes
LONGvariable-lengthcharacterdatatypeupto2GB
RAWrawbinarydatatypeupto2000bytes
longrawprimitivebinarydatatype,thelongestis2GB
A12-byteaddressintherawdatabase,usedtolocatearowonthediskandtheobjectitpointsto
urawidgeneralTherawid,usedinthetable,isarepresentationoftherowprimarykey
largeobject
bloblargeprimitivebinaryobject,themaximumlengthis4GB
cloblargecharacterobject,Themaximumlengthis4GB
nclobThelargecharacterdatacomposedofNLScharactersetcharacters,themaximumlengthis4GB
bfileprovidessupportforoperatingsystemBinaryfileaccessinthefile,maximum4GB
Datetype
dateA7-bytedate/timedatatypewithfixedwidth,including7attributes:century,year,Month,day,hour,minute,second
timestamp7-byteor12-bytefixed-widthdate/timedatatype,whichcancontaindecimalsecondswhenitisdifferentfromDATE,andthedecimalpointcanretainupto9digits
timestampwithtimezornprovidessupportfortimezone
timestampwithlocaltimezornisatimezonesensitivetimetype
interalyeartomonthstorageoftheyearandmonth,canbeAddandsubtractdateandtimestamptypes
intervaldaytosecondstoresdays/hours/minutes/seconds,andsupports9decimalseconds
datatype
number[(precision[,scale])]Range1~38
Thenumberofdigitsintheprecisionnumber
Thenumberofdigitstotherightofthedecimalpoint
/p>
oralceonlyprovidesonenumbertype:NUMBERtype,storedinoracleMostofthestoreddigitaldatacanberegardedasnumbertype(supportbinary_float,binary_doubleafter10g).ThefollowingdatatypesarecompatiblewithANSISQL:
/**
ANSISQL:AmericanNationalStandardsInstitute(ANSI)"isanorganizationthatapprovesmultipleindustrystandards.SQLasarelationshipThestandardlanguageusedbythetypedatabasewasoriginallyapprovedin1986basedontheimplementationofIBM.In1987,the"InternationalOrganizationforStandardization(ISO)"adoptedANSISQLasaninternationalstandard.Thisstandardwasrevisedin1992(SQL-92),revisedagainin1999(SQL-99).ThelatestoneisSQL-2003.
**/
float
numberic
decimal
number
integer
int
smallint
doubleprecision
real
Researchdirection
Thequalityoftheoriginaldatahasagreatrelationshipwiththelaterdatareadingandprocessing.NowtheresearchontheoriginaldataMainlyintermsofdataacquisitionanddatacompression,manymethodshavebeenproposedinthisregard.Forexample,theSARoriginaldatacompressionalgorithmbasedonliftingwaveletbelongstotransformcoding.Unlikedirectcodingmethods,transformcodingfirsttransformstheoriginaldataandthenQuantizationcodingtransformcoefficients.Thepurposeoftransformistomapthespatialdomainpixelarrayintoanuncorrelatedandcompactenergydistributiontransformcoefficientarray.Atthistime,thelargetransformcoefficientsthataccountfortheminorityrepresentthemostimportantenergycomponentsoftheimage,andthemajorityareThesmalltransformcoefficientsrepresentsomeunimportantdetailcomponents.Thedetailcomponentsrepresentedbythesmallcoefficientsareremovedbyquantization,andasmallamountofcodewordsareusedtodescribethemainenergyrepresentedbythelargecoefficientstoachieveahighcompressionratio.