Relational Calculus Essay Example for Free

Relational Calculus EssayIntroduction Procedural Query talking to doubtfulness specification involves giving a step by step process of obtaining the query resolvent e.g., sexual coitional algebra usage c in alls for detailed knowledge of the operators involved difficult for the use of non-experts Declarative Query vocabulary query specification involves giving the logical marks the results are required to satisfy easy for the use of non-experts professor P Sreenivasa Kumar, surgical incision of CSE, IITM. 1TRC a declarative query languageTuple shifting associated with a relation back ( called the redact relation ) takes tuples from the range relation as its respects t tuple variable over relation r with scheme R(A,B,C ) t. A stands for value of column A etc.TRC Query basic form t1.Ai1, t2.Ai2,tm.Aim allege calculus materialization involving tuple variables t1, t2,, tm, tm+1,,ts specifies the condition to be satisfied professor P Sreenivasa Kumar, Departm ent of CSE, IITM. 2An example TRC query student (rollNo, name, degree, year, sex, deptNo, advisor ) department (deptId, name, hod, phone ) Obtain the rollNo, name of all miss students in the maths Dept (deptId = 2) s.rollNo,s.name student(s) s.sex=F s.deptNo=2 attributes required in the resultThis predicate is true up whenever value of s is a tuple from the student relation, false otherwise In general, if t is a tuple variable with range relation r, r( t ) is taken as a predicate which is true if and only if the value of t is a tuple in r professor P Sreenivasa Kumar, Department of CSE, IITM. 3General form of the condition in TRC queriesAtomic expressions are the following 1. r ( t ) -true if t is a tuple in the relation instance r 2. t1. Ai t2 .Aj compOp is one of , , =, 3. t.Ai c c is a constant of appropriate grapheme Composite expressions 1. Any atomic expression 2. F1 F2 ,, F1 F2 , F1 where F1 and F2 are expressions 3. (t) (F), (t) (F) where F is an expression and t i s a tuple variable Free Variables Bound Variables quantified variables professor P Sreenivasa Kumar, Department of CSE, IITM. 4Interpretation of the query in TRC completely possible tuple assignments to the free variables in the query are considered. For any specific assignment, if the expression to the right of the straight mensuration evaluates to true, that combination of tuple values would be used to produce a tuple in the result relation. composition producing the result tuple, the values of the attributes for the corresponding tuple variables as specified on the left side of the vertical bar would be used. Note The only free variables are the ones that appear to the left of the vertical bar prof P Sreenivasa Kumar, Department of CSE, IITM. 5Example TRC queriesObtain the rollNo, name of all girl students in the Maths Dept s.rollNo,s.name student(s) s.sex=F ( d)(department(d) d.name=Maths d.deptId = s.deptNo)s free tuple variabled existentially bound tuple variableExis tentially or universally quantified tuple variables can be used on the RHS of the vertical bar to specify query conditions Attributes of free (or unbound ) tuple variables can be used on LHS of vertical bar to specify attributes required in the results Prof P Sreenivasa Kumar, Department of CSE, IITM. 6Example Relational Schemestudent (rollNo, name, degree, year, sex, deptNo, advisor) department (deptId, name, hod, phone) professor (empId, name, sex, startYear, deptNo, phone) track down (courseId, cname, credits, deptNo) enrollment (rollNo, courseId, sem, year, grade) teaching (empId, courseId, sem, year, classRoom) preRequisite (preReqCourse, courseID) Prof P Sreenivasa Kumar, Department of CSE, IITM. 7Q2 Q3 Q4 Q5Example queries in TRC (1/5)1)Determine the departments that do not have any girl studentsstudent (rollNo, name, degree, year, sex, deptNo, advisor) department (deptId, name, hod, phone) d.namedepartment(d) ( s)(student(s) s.sex =F s.deptNo = d.deptId)Prof P Sreeniva sa Kumar, Department of CSE, IITM.8Examples queries in TRC (2/5)Schema2)Obtain the names of courses enrolled by student named Mahesh c.name course(c) (s) (e) ( student(s) enrollment(e) s.name = Mahesh s.rollNo = e.rollNo c.courseId = e.courseId Prof P Sreenivasa Kumar, Department of CSE, IITM.9Examples queries in TRC (3/5)Schema3) occupy the names of students who have scored S in all subjects they have enrolled. Assume that all(prenominal) student is enrolled in at least one course. s.name student(s) (e)(( enrollment(e) e.rollNo = s.rollNo) e.grade =S)person P with all S grades for enrollment tuples not having her roll number, LHS is false for enrollment tuples having her roll number, LHS is true, RHS also true so the price reduction is true for all e tuples person Q with some non-S grades for enrollment tuples not having her roll number, LHS is false for enrollment tuples having her roll number, LHS is true, but RHS is false for at least one tuple. So the implication is not true for at least one tuple. Prof P Sreenivasa Kumar, Department of CSE, IITM. 10Examples queries in TRC (4/5)Schema4) Get the names of students who have taken at least one course taught by their advisor s.name student(s) (e)(t)(enrollment(e) teaching(t) e.courseId = t.courseId e.rollNo = s.rollNo t.empId = s.advisor 5) ostentation the departments whose HODs are teaching at least one course in the current semester d.name department(d) (t)(teaching(t) t.empid = d.hod t.sem = odd t.year = 2008) Prof P Sreenivasa Kumar, Department of CSE, IITM. 11Examples queries in TRC (5/5)Schema6)Determine the students who are enrolled for every course taught by Prof Ramanujam. Assume that Prof Ramanujam teaches at least one course. 1. s.rollNo student (s) 2. (c)(course (c) 3. ((t),(p)( teaching(t) professor(p) 4. t.courseId = c.courseId 5. p.name = Ramanujam 6. p.empId = t.empId )) 7. (e) (enrollment(e) 8. e.courseId = c.courseId 9. e.rollNo = s.rollNo) 10. ) 11. Prof P Sr eenivasa Kumar, Department of CSE, IITM. 12Problem with unrestricted use of NegationWhat is the result of the querys.rollNo student(s) ?Infinite answers Unsafe TRC expression Any expression whose result uses constants / values that do not appear in the instances of any of the database relations. Unsafe expressions are to be avoided while specifying TRC queries.Prof P Sreenivasa Kumar, Department of CSE, IITM.13Expressive power of TRC and Relational AlgebraIt can be shown that both Tuple Relational Calculus and Relational Algebra have the same expressive power A query can be formulated in (safe) TRC if and only if it can be formulated in RA Both can not be used to formulate queries involving transitive closure find all direct or indirect pre-requisites of a course find all subordinates of a specific employee etc.Prof P Sreenivasa Kumar, Department of CSE, IITM.