Relational Database Model

Transkript

1 Database Systems Unit 3 Relational Database Model DB / PRM1 / db04_relationalmodel.ppt / Version Learning Goals In this unit you will learn what the relational database model is what componentes are used in the relational model how entities are represented in relational databases how relationships are represented in relational databases DB / PRM1 / db04_relationalmodel.ppt / Version

2 Characteristics The relational model was proposed by Dr. E. F. Codd of IBM in the paper "A Relational Model for Large Shared Data Banks," Communications of the ACM, June The model proposed by Codd is a theoretical and formal model, based on the ideas of sets. In the early 1980s, the first commercial implementations of a relational DBMS were available, such as Oracle DBMS and the SQL/DS system on the MVS operating system by IBM. Today, most of the actually commercially available DBMS are based on the relational database model (RDM). DB / PRM1 / db04_relationalmodel.ppt / Version Relation Student Id Name Vorname Adresse PLZ Ort BLANJ Blanc Jean-Pierre Ch. du Crêt Bienne MEIEP1 Meier Peter Unterstr Biel MUSTP Muster Hans Oberstr Biel Tel RENAJ Renard Joceline Rue du Lac Nidau DB / PRM1 / db04_relationalmodel.ppt / Version

3 Informal and Formal Terms Informal Terms Table Column, Attribute, Property Row, Record, Dataset Possible values in a column Table Definition Database Definition Formal Terms Relation Attribute / Domain Tuple Domain Relation Schema Relational Database Schema DB / PRM1 / db04_relationalmodel.ppt / Version Domains A domain D is a set of atomic values, that is each value in the domain is indivisible as far as the relational model is concerned. A common method of specifying a domain is to specify a data type. It is also useful to specify a name for the domain, to help in interpreting values. DB / PRM1 / db04_relationalmodel.ppt / Version

4 Domain: Formal Definition The domain of an attribute is the set of the possible values of this attribute. A format is also specified for each domain. A domain is given a name, data type and format. It may have also have a format defined for it. Furthermore, a domain may be limited by constraints. An attribute designates a role or interpretations for the domain: For example, the same domain local phone numbers plays the role of the home phone or of the office phone. DB / PRM1 / db04_relationalmodel.ppt / Version Types of Domains We distinguish between two types of domains: Static: A static domain is a predefined set of values, where the values are constant for a certain time period or forever. Dynamic: A dynamic domain is a set of time- varying values. The set grows or shrinks dynamically and is not constant! Domains are part of the database structure and their definitions and should therefore be stored in the database system and not being defined in applications and functions! However, some DBMS only have limited possibilities to support this. DB / PRM1 / db04_relationalmodel.ppt / Version

5 Relation Schema: Formal Definition A relation schema R, denoted by R(A 1, A 2,, A n ) is made up of a relation name R and a List of attributes A 1, A 2,, A n Each attribute A i is the name of a role played by some domain D in the relation schema R. D i is called the domain of A i, also denoted by dom(a i ) A relation schema R is used to describe a relation where R is the name of the relation. The number of attributes n is called the degree of a relation. DB / PRM1 / db04_relationalmodel.ppt / Version Relation Schema: Formal Definition Example: STUDENT(ID, LastName, FirstName, StreetAddress, CityNumber, CityName, HomePhone) Using the data type, the definition is sometimes written: STUDENT(ID: string, LastName: string, FirstName: string, StreetAddress: string, CityNumber: integer, CityName: string, HomePhone: string) DB / PRM1 / db04_relationalmodel.ppt / Version

6 Relation: Formal Definition A relation r (or relation state r) of the relational schema R(A 1, A 2, A n ), also denoted by r(r), is a set of n- tuples (or tuples) r = {t 1, t 2, t m } DB / PRM1 / db04_relationalmodel.ppt / Version Tuple: Formal Definition A tuple is an (ordered) set of values Each tuple t is an ordered list of n values t = <v 1, v 2,, v n >, where each value v i,1 i n, is an element of dom(a i ) or is a special NULL value. The i-th value of a tuple corresponds to the attribute A i is referred to as t[a i ] or shortly t[i]. DB / PRM1 / db04_relationalmodel.ppt / Version

7 NULL Value Additionally to domain values, attributes may or may not accept NULL values. A NULL value is not simply the number of 0 (or an empty character string); it has to be distinguished clearly from "0". NULL values are hardly disputed in the professional world, because their meaning (semantics) is not always very clear: Value not applicable for a given tuple Value not know (but may exist) Value known to exist, but not (yet) available DB / PRM1 / db04_relationalmodel.ppt / Version Relational Database Schema: Formal Definition A Relational Database Schema is a set S of relation schemas R i that belong to the same database and a set of integrity constraints IC. S is the name of the database and written by S = {R 1, R 2,..., R n } A relational database is a database that is perceived by the user as a collection of time-varying, normalized relations of assorted degrees! E. F. Codd DB / PRM1 / db04_relationalmodel.ppt / Version

8 Relational Database Schema: Formal Definition A relational database state DB of S is a set of relation states DB = {r 1, r 2,..., r n } such that r i is a state of R i such that the r i relation state satisfy the integrity constraint specified in IC. A database state that does not obey all the integrity constraints is called an invalid state, and a state that satisfies all the constraints is called a valid state. Each relational DBMS must have a data definition language (DDL) for defining a relational database schema (mostly SQL). DB / PRM1 / db04_relationalmodel.ppt / Version Relational Integrity Constraints Constraints are conditions that must hold on all valid relation instances. There are three main types of constraints: 1. Key constraints 2. Entity integrity constraints 3. Referential integrity constraints Additionally, most DBMS allow to specify user defined integrity constraints. DB / PRM1 / db04_relationalmodel.ppt / Version

9 Key Constraints Superkey of R: A set of attributes SK of R such that no two tuples in any valid relation instance r(r) will have the same value for SK. That is, for any distinct tuples t1 and t2 in r(r), t1[sk] t2[sk] Key of R: A "minimal" superkey; that is, a superkey K such that removal of any attribute from K results in a set of attributes that is not a superkey If a relation has several (candidate) keys, one is chosen arbitrarily to be the primary key DB / PRM1 / db04_relationalmodel.ppt / Version Entity Integrity Entity Integrity: The primary key attributes PK of each relation schema R in S cannot have null values in any tuple of r(r). This is because primary key values are used to identify the individual tuples. t[pk] null for any tuple t in r(r) Note: There may be other attributes of R may to be similarly constrained to be not Null, even though they are not members of the primary key. DB / PRM1 / db04_relationalmodel.ppt / Version

10 Referential Integrity A constraint involving two relations Used to specify a relationship among tuples in two relations: the referencing relation and the referenced relation Tuples in the referencing relation R 1 have attributes FK (called foreign key attributes) that reference the primary key attributes PK of the referenced relation R 2. A tuple t 1 in R 1 is said to reference a tuple t 2 in R 2 if t 1 [FK] = t 2 [PK] A referential integrity constraint can be displayed in a relational database schema as a directed arc from R 1.FK to R 2 or R 2.PK resp. DB / PRM1 / db04_relationalmodel.ppt / Version Referential Integrity Constraint The value in the foreign key column (or columns) FK of the the referencing relation R 1 can be either: 1. a value of an existing primary key value of the corresponding primary key PK in the referenced relation R 2,, or 2. NULL In case (2), the FK in R 1 may not be a part of its own primary key DB / PRM1 / db04_relationalmodel.ppt / Version

11 Graphical Representation: Database Schema Diagram DB / PRM1 / db04_relationalmodel.ppt / Version Textual Representation Employee (SSN, FName, Minit, Lname,, PK(SSN) ) Dependent (ESSN, Dependent_Name, Bdate,, PK(ESSN, Dependent_Name), FK(ESSN) Employee) ) Additionally, we may specify for each attribute whether it accepts NULL values or not (Ex: "N" or "NN") DB / PRM1 / db04_relationalmodel.ppt / Version

12 Update Operations The operations of the relational model can be categorized into retrievals and updates. There are three update operations: insert, delete and modify. Insert is used to insert a new tuple or tuples in a relation. Delete is used to delete tuples. Update (or modify) is used to change the values of some attributes in existing tuples. Whenever these operations are applied, the integrity constraints specified on the relational database schema should not be violated. DB / PRM1 / db04_relationalmodel.ppt / Version Update Operations Exercise: Which type of integrity constraints can be violated by the update operations above? DB / PRM1 / db04_relationalmodel.ppt / Version