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Relational Data Model a Brief History of Data Models Relational Model A Brief History of Data Models 1950s file systems, punched cards 1960s hierarchical Relational Data Model IMS 1970s network CODASYL, IDMS 1980s relational INGRES, ORACLE, DB2, Sybase Paradox, dBase 1990s object oriented and object relational O2, GemStone, Ontos Relational Model COURSE Sets Courseno Subject Lecturer Machine collections of items of the same type no order domain range CS250 Programming Lindsey Sun no duplicates 1:many CS260 Graphics Hubbold Sun Mappings many:1 CS270 Micros Woods PC CS290 Verification Barringer Sun 1:1 many:many Relational Model Notes Comparative Terms no duplicate tuples in a relation a relation is a set of tuples Formal Oracle no ordering of tuples in a relation Relation schema Table description a relation is a set Relation Table Tuple Row attributes of a relation have an implied Attribute Column ordering Domain Value set but used as functions and referenced by name, not position Notation every tuple must have attribute values drawn Course (courseno, subject, from all of the domains of the relation or the equipment) special value NULL Student(studno,name,hons) Enrol(studno,courseno,labmark) all a domain’s values and hence attribute’s values are atomic. 1 Keys Foreign Key SuperKey a set of attributes in a relation that exactly a set of attributes whose values together uniquely matches a (primary) key in another relation identify a tuple in a relation the names of the attributes don’t have to be the Candidate Key same but must be of the same domain a superkey for which no proper subset is a a foreign key in a relation A matching a primary superkey…a key that is minimal . key in a relation B represents a Can be more than one for a relation many:one relationship between A and B Primary Key a candidate key chosen to be the main key for the Student(studno,name,tutor,year) relation. One for each relation Staff(lecturer,roomno,appraiser) Keys can be composite STUDENT studno name hons tutor year Referential Integrity s1 jones ca bush 2 s2 brown cis kahn 2 Student(studno,name,tutor,year) s3 smith cs goble 2 s4 bloggs ca goble 1 Staff(lecturer,roomno,appraiser) s5 jones cs zobel 1 s6 peters ca kahn 3 CASCADE STAFF delete all matching foreign key tuples lecturer roomno appraiser kahn IT206 watson eg. STUDENT bush 2.26 capon RESTRICT goble 2.82 capon zobel 2.34 watson can’t delete primary key tuple STAFF whilst a watson IT212 barringer foreign key tuple STUDENT matches woods IT204 barringer capon A14 watson NULLIFY lindsey 2.10 woods foreign key STUDENT.tutor set to null if the barringer 2.125 null foreign key ids allowed to take on null Entity Integrity and Nulls Relational Model No part of a key can be null General Attribute values THREE categories of null Simple Atomic values 1. Not applicable Flexible Known domain 2. Not known Easy to query declaratively without Sometimes can be null 3. Absent (not recorded) programming STUDENT But..... studno name hons tutor year thesis title Good design essential s1 jones ca bush 2 null s2 brown cis kahn 2 null Integrity essential s3 smith null goble 2 null s4 bloggs ca goble 1 null Poor semantics s5 jones cs zobel 1 null s6 peters ca kahn 3 null Relationships based on ‘value-matching’ 2 Relational Design Informal guidelines stud name tutor roomno course lab subject Semantics of the attributes no no mark easy to explain relation s1 jones bush 2.26 cs250 65 programming s1 jones bush 2.26 cs260 80 graphics doesn’t mix concepts s1 jones bush 2.26 cs270 47 electronics s2 brown kahn IT206 cs250 67 programming Reducing the redundant values in tuples s2 brown kahn IT206 cs270 65 electronics s3 smith goble 2.82 cs270 49 electronics Choosing attribute domains that are atomic s4 bloggs goble 2.82 cs280 50 design s5 jones zobel 2.34 cs250 0 programming Reducing the null values in tuples s6 peters kahn IT206 cs250 2 programming null null capon A14 null null null Disallowing spurious tuples null null null null cs290 null specification s7 patel null null null null null Definitions (N)-tuple a set of (n) attribute-value pairs representing a single instance Cartesian Product The cartesian product (×) between n sets is the set of a relation’s mapping between its domains. of all possible combinations of the elements of those sets. Degree the number of attributes a relation has. Domain set of all possible values for an attribute; for attribute A, the Cardinality a number of tuples a relation has. domain is represented as dom(A). A domain has a format and a base data type. Roles several attributes can have the same domain; the attributes indicate different roles in the relation. Relation Schema denoted by R(A1, A2, …, An), is made up of relation name R and list of attributes A , A , …, A . Key (SuperKey) a set of attributes whose values together uniquely 1 2 n identify every tuple in a relation. Let t1 and t2 be two tuples on relation Relation a subset of the cartesian product of its domains. Given a r of relation schema R, and sk be a set of attributes whose values are the relation schema R, a relation on that schema r, a set of attributes A1..An key for the relation schema R, then t1[sk] ≠ t2[sk]. for that relation then (Candidate) Key a (super)key that is minimal, i.e. has no proper ⊆ × × × r(R) (dom(A1) dom(A2) ... dom(An)) subsets that still uniquely identify every tuple in a relation. There can be several for one relation. Attribute a function on a domain for each instance of the mapping or tuple Primary Key a candidate key chosen to be the main key for the relation. There is only one for each relation. Attribute Value the result of the attribute function. Each instance of the mapping is represented by one attribute value drawn from each Foreign Key a candidate key of relation A situated in relation B. domain or a special NULL value. Given a tuple t and an attribute A for Database a set of relations. a relation r, t[A]--> a, where a is the attribute’s value for that tuple. 3.
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