ORACLE官方基础教程第10讲(经典版)

IncludingConstraints

ObjectivesAftercompletingthislesson,youshouldbeabletodothefollowing:DescribeconstraintsCreateandmaintainconstraintsWhatareConstraints?Constraintsenforcerulesatthetablelevel.Constraintspreventthedeletionofatableiftherearedependencies.Thefollowingconstrainttypesarevalid:NOTNULLUNIQUEPRIMARYKEYFOREIGNKEYCHECKConstraintGuidelinesNameaconstraintortheOracleservergeneratesanamebyusingtheSYS_Cn

format.Createaconstrainteither:Atthesametimeasthetableiscreated,orAfterthetablehasbeencreatedDefineaconstraintatthecolumnortablelevel.Viewaconstraintinthedatadictionary.DefiningConstraintsCREATETABLE[schema.]table (column

datatype[DEFAULTexpr] [column_constraint], ... [table_constraint][,...]);CREATETABLEemployees( employee_idNUMBER(6), first_nameVARCHAR2(20), ... job_idVARCHAR2(10)NOTNULL, CONSTRAINTemp_emp_id_pk PRIMARYKEY(EMPLOYEE_ID));DefiningConstraintsColumnconstraintlevel

Tableconstraintlevelcolumn[CONSTRAINTconstraint_name]constraint_type,column,...

[CONSTRAINTconstraint_name]constraint_type(column,...),TheNOT

NULLConstraintEnsuresthatnullvaluesarenotpermittedforthecolumn:NOTNULLconstraint(Norowcancontain

anullvaluefor

thiscolumn.)AbsenceofNOTNULLconstraint(Anyrowcancontain

nullforthiscolumn.)NOTNULL

constraint…CREATETABLEemployees(employee_idNUMBER(6),last_nameVARCHAR2(25)NOTNULL,salaryNUMBER(8,2),commission_pctNUMBER(2,2),hire_dateDATECONSTRAINTemp_hire_date_nnNOTNULL,...TheNOT

NULLConstraintIsdefinedatthecolumnlevel:

SystemnamedUsernamedTheUNIQUEConstraintEMPLOYEESUNIQUEconstraintINSERTINTONotallowed:alreadyexistsAllowed…TheUNIQUEConstraintDefinedateitherthetablelevelorthecolumnlevel:CREATETABLEemployees(employee_idNUMBER(6),last_nameVARCHAR2(25)NOTNULL,emailVARCHAR2(25),salaryNUMBER(8,2),commission_pctNUMBER(2,2),hire_dateDATENOTNULL,...CONSTRAINTemp_email_ukUNIQUE(email));ThePRIMARY

KEYConstraintDEPARTMENTS

PRIMARYKEYINSERTINTONotallowed(Nullvalue)Notallowed(50alreadyexists)…CREATETABLEdepartments(department_idNUMBER(4),department_nameVARCHAR2(30)CONSTRAINTdept_name_nnNOTNULL,manager_idNUMBER(6),location_idNUMBER(4),CONSTRAINTdept_id_pkPRIMARYKEY(department_id));ThePRIMARY

KEYConstraintDefinedateitherthetablelevelorthecolumnlevel:TheFOREIGN

KEYConstraintDEPARTMENTS

EMPLOYEESFOREIGN

KEYINSERTINTONotallowed

(9doesnotexist)AllowedPRIMARY

KEY……TheFOREIGN

KEYConstraintDefinedateitherthetablelevelorthecolumnlevel:CREATETABLEemployees(employee_idNUMBER(6),last_nameVARCHAR2(25)NOTNULL,emailVARCHAR2(25),salaryNUMBER(8,2),commission_pctNUMBER(2,2),hire_dateDATENOTNULL,...department_idNUMBER(4),CONSTRAINTemp_dept_fkFOREIGNKEY(department_id)REFERENCESdepartments(department_id),CONSTRAINTemp_email_ukUNIQUE(email));FOREIGN

KEYConstraint

KeywordsFOREIGNKEY:DefinesthecolumninthechildtableatthetableconstraintlevelREFERENCES:IdentifiesthetableandcolumnintheparenttableONDELETECASCADE:Deletesthedependentrowsinthechildtablewhenarowintheparenttableisdeleted.ONDELETESETNULL:ConvertsdependentforeignkeyvaluestonullTheCHECKConstraintDefinesaconditionthateachrowmustsatisfyThefollowingexpressionsarenotallowed:ReferencestoCURRVAL,NEXTVAL,LEVEL,andROWNUMpseudocolumnsCallstoSYSDATE,UID,USER,andUSERENVfunctionsQueriesthatrefertoothervaluesinotherrows...,salary NUMBER(2)CONSTRAINTemp_salary_minCHECK(salary>0),...AddingaConstraintSyntaxUsetheALTERTABLEstatementto:Addordropaconstraint,butnotmodifyitsstructureEnableordisableconstraintsAddaNOTNULLconstraintbyusingtheMODIFYclauseALTERTABLE tableADD[CONSTRAINTconstraint]type(column);AddingaConstraintAddaFOREIGNKEYconstrainttotheEMPLOYEES

tableindicatingthatamanagermustalreadyexistasavalidemployeeintheEMPLOYEEStable.ALTERTABLEemployeesADDCONSTRAINTemp_manager_fkFOREIGNKEY(manager_id)REFERENCESemployees(employee_id);Tablealtered.DroppingaConstraintRemovethemanagerconstraintfromtheEMPLOYEEStable.RemovethePRIMARYKEYconstraintontheDEPARTMENTStableanddroptheassociatedFOREIGNKEYconstraintontheEMPLOYEES.DEPARTMENT_IDcolumn.ALTERTABLEemployeesDROPCONSTRAINTemp_manager_fk;Tablealtered.ALTERTABLE departmentsDROPPRIMARYKEYCASCADE;Tablealtered.DisablingConstraintsExecutetheDISABLEclauseoftheALTER

TABLEstatementtodeactivateanintegrityconstraint.ApplytheCASCADEoptiontodisabledependentintegrityconstraints.ALTERTABLE employeesDISABLECONSTRAINT emp_emp_id_pkCASCADE;Tablealtered.EnablingConstraintsActivateanintegrityconstraintcurrentlydisabledinthetabledefinitionbyusingtheENABLEclause.

AUNIQUEorPRIMARYKEYindexisautomaticallycreatedifyouenableaUNIQUEkeyorPRIMARYKEYconstraint.ALTERTABLE employeesENABLECONSTRAINT emp_emp_id_pk;Tablealtered.CascadingConstraintsTheCASCADECONSTRAINTSclauseisusedalongwiththeDROPCOLUMNclause.TheCASCADECONSTRAINTSclausedropsallreferentialintegrityconstraintsthatrefertotheprimaryanduniquekeysdefinedonthedroppedcolumns.TheCASCADECONSTRAINTSclausealsodropsallmulticolumnconstraintsdefinedonthedroppedcolumns.CascadingConstraintsExample:ALTERTABLEtest1DROP(pk)CASCADECONSTRAINTS;Tablealtered.ALTERTABLEtest1DROP(pk,fk,col1)CASCADECONSTRAINTS;Tablealtered.SELECT constraint_name,constraint_type, search_conditionFROM user_constraintsWHERE table_name='EMPLOYEES';ViewingConstraintsQuerythe