5.4 What is the difference between a key and a superkey.Ans:Any key is a superkey (but not vice versa)Any set of attributes that includes a key is a superkeyA minimal superkey is also a key5.6 Discuss the characteristics of relations that make them different from ordinary tables and files.Ans:Tables and files have some particular order but relations don’t.5.8 Discuss the entity integrity and referential integrity constraints. Why is each considered important?Ans:Integrity integrity constraints: The entity integrity constraints states that no primary key value can be NULL. This is because the primary key value is used to identify individual tuples in a relation. Having NULL values for the primary keyimplies that we cannot identify some tuples.referential integrity constraints: It is specified between two relations and is used to maintain the consistency among tuples in the two relations.5.9 Define foreign key. What is this concept used for?Ans:1.The attributes in FK have the same domain(s) as primary key attributes PK of R2; the attributes FK are said to reference or refer to the relation R2.2.A value of FK in a tuple t1 of the current state r1(R1) either occurs as a value of PK for some tuple t2 in the current state r2(R2) or is NULL. In the former case, we have t1[FK] = t2 [PK], and we say that the tuple t1 references or refersto the tuple t2.FK is used to define referential integrity constraints.5.11 Suppose each of the following update operations is applied directly to the databaseof Figure 5.6. Discuss all integrity constraints violated by each operation, if any,and the different ways of enforcing these constraints:(a) Insert < 'Robert', 'F', 'Scott', '943775543', '21-JUN-42', '2365 Newcastle Rd,Bellaire, TX', M, 58000, '888665555', 1 > into EMPLOYEE.(b) Insert < 'ProductA', 4, 'Bellaire', 2 > into PROJECT.(c) Insert < 'Production', 4, '943775543', '01-OCT-88' > into DEPARTMENT.(d) Insert < '677678989', null, '40.0' > into WORKS_ON.(e) Insert < '453453453', 'John', M, '12-DEC-60', 'SPOUSE' > into DEPENDENT.(f) Delete the WORKS_ON tuples with ESSN= '333445555'.(g) Delete the EMPLOYEE tuple with SSN= '987654321'.(h) Delete the PROJECT tuple with PNAME= 'ProductX'.(i) Modify the MGRSSN and MGRSTARTDATE of the DEPARTMENT tuple with DNUMBER=5 to '123456789' and '01-OCT-88', respectively.(j) Modify the SUPERSSN attribute of the EMPLOYEE tuple with SSN= '999887777' to'943775543'.(k) Modify the HOURS attribute of the WORKS_ON tuple with ESSN= '999887777' andPNO= 10 to '5.0'.Answers:(a) No constraint violations.(b) Violates referential integrity because DNUM=2 and there is no tuple in the DEPARTMENT relation with DNUMBER=2. We may enforce the constraint by: (i) rejecting the insertion of the new PROJECT tuple, (ii) changing the value of DNUM in the newPROJECT tuple to an existing DNUMBER value in the DEPARTMENT relation, or (iii)inserting a new DEPARTMENT tuple with DNUMBER=2.(c) Violates both the key constraint and referential integrity. Violates the key constraintbecause there already exists a DEPARTMENT tuple with DNUMBER=4. We may enforcethis constraint by: (i) rejecting the insertion, or (ii) changing the value of DNUMBERin the new DEPARTMENT tuple to a value that does not violate the key constraint. Violatesreferential integrity because MGRSSN='943775543' and there is no tuple in theEMPLOYEE relation with SSN='943775543'. We may enforce the constraint by: (i)rejecting the insertion, (ii) changing the value of MGRSSN to an existing SSN value inEMPLOYEE, or (iii) inserting a new EMPLOYEE tuple with SSN='943775543'.(d) Violates both the entity integrity and referential integrity. Violates entity integritybecause PNO, which is part of the primary key of WORKS_ON, is null. We may enforcethis constraint by: (i) rejecting the insertion, or (ii) changing the value of PNO in thenew WORKS_ON tuple to a value of PNUMBER that exists in the PROJECT relation.Violates referential integrity because ESSN='677678989' and there is no tuple in theEMPLOYEE relation with SSN='677678989'. We may enforce the constraint by: (i)rejecting the insertion, (ii) changing the value of ESSN to an existing SSN value inEMPLOYEE, or (iii) inserting a new EMPLOYEE tuple with SSN='677678989'.(e) No constraint violations.(f) No constraint violations.(g) Violates referential integrity because several tuples exist in the WORKS_ON,DEPENDENT, DEPARTMENT, and EMPLOYEE relations that reference the tuple beingdeleted from EMPLOYEE. We may enforce the constraint by: (i) rejecting the deletion, or(ii) deleting all tuples in the WORKS_ON, DEPENDENT, DEPARTMENT, and EMPLOYEErelations whose values for ESSN, ESSN, MGRSSN, and SUPERSSN, respectively, is equalto'987654321'.(h) Violates referential integrity because two tuples exist in the WORKS_ON relations thatreference the tuple being deleted from PROJECT. We may enforce the constraint by: (i)rejecting the deletion, or (ii) deleting the tuples in the WORKS_ON relation whose valuefor PNO=1 (the value for the primary key PNUMBER for the tuple being deleted fromPROJECT).(i) No constraint violations.(j) Violates referential integrity because the new value of SUPERSSN='943775543' andthere is no tuple in the EMPLOYEE relation with SSN='943775543'. We may enforce theconstraint by: (i) rejecting the deletion, or (ii) inserting a new EMPLOYEE tuple withSSN='943775543'.(k) No constraint violations.5.13 Consider the relation CLASS(Course#, Univ_Section#, InstructorName, Semester, BuildingCode, Room#, TimePeriod, Weekdays, CreditHours). This represents classes taught in a university with unique Univ_Section#. Give what you think should be various candidate keys and write in your own words under what constraints each candidate key would be valid.Answer:Possible candidate keys include the following (Note: We assume that the valuesof theSemester attribute include the year; for example "Spring/94" or "Fall/93" could bevalues for Semester):1. {Semester, BuildingCode, Room#, TimePeriod, Weekdays} if the same room cannot beused at the same time by more than one course during a particular semester.2. {Univ_Section#} if it is unique across all semesters.3. {InstructorName, Semester} if an instructor can teach at most one course during eachsemester.4. If Univ_Section# is not unique, which is the case in many universities, we have toexamine the rules that the university uses for section numbering. For example, if thesections of a particular course during a particular semester are numbered 1, 2, 3,
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