The Relational Model 1

1. Lecture 5 The Relational Model 1 Prof. Sin-Min Lee Department of Mathematics and Computer Science Prof. Sin-Min Lee Department of Computer Science

2. History of Relational Model • First proposed by a E.F. Codd in 1970. Codd proposed the Relational model in 1970. • “ A relational model of data for large shared data banks.” • He linked the representation of data with that of mathematical sets. • First research started at IBM’s San Jose Research Laboratory. Prototype was called System R. • Commercial RDBMS’s started to appear in late 1970’s and early 1980’s. Most well known is Oracle.

3. Codd’s Original Paper • “A Relational Model of Data for Large Shared Data Banks” • Communications of the ACM, Volume 13, Number 6, June 1970 • Lower level (basement) of the new Martin Luther King, Jr. Library • Get to roll the shelves apart to access journals.

4. Codd’s Reasons • Data independence from database implementation such as machine representation • Natural structure of data • Can be analyzed mathematically (Codd was a mathematician by training)

5. Alternative: Network Model • Charles A. Bachman 1973 ACM Turing Award Lecture “The Programmer as Navigator” • Communications of the ACM, Volume 16, Number 11, November 1973, pp. 653-657

8. Data Models • Codd suggests that any data model has three components: the data structures; the integrity constraints; the data manipulation operators.

10. Relational Data Structure Relation Attribute EMPLOYEE Emp# Name Emp Sex Mgr Emp# E1 Jones Male E65 E6 Smith Male E28 E28 Jones Female - Heading Body Female Male Gender Domain

11. The Domain Employee Emp# E1 E2 E3 Name Red Brown Black Mgr# - E1 E1 Attributes Person Name E1, E2, E3,E4 Red, Brown Black, Blue Domains Emp#

14. Seven Characteristics of a relation • The name of the relation is different from all others. • Each cell of the relation contains only one value • Each attribute (or field) has a name that is distinct. • All the values of a particular attribute are from the same domain. • The order of the attributes makes no difference. • There are no duplicate tuples • The order of the tuples makes no difference.

15. Example of the Student table. Primary Key

16. Other terms... • Cardinality = Number of rows • Degree = Number of columns Degree = 5 Cardinality = 6

24. The Relational Data Model • DATA STRUCTURES - domain, attribute, relation, tuple, primary key, degree, cardinality. • INTEGRITY CONSTRAINTS - entity integrity and referential integrity. • DATA MANIPULATION OPERATIONS - defined through relational algebra and equivalent relational calculus.

25. Entity Integrity • No component of the Primary Key of a base relation is allowed to accept nulls. Surname Red Black Red Black Given Name John Fred Salary $40,000 $50,000 $60,000 $70,000

33. Foreign Key • A foreign key is an attribute or attribute combination of one relation R2 whose values are required to match those of the primary key of relation R1 where R1 and R2 are not necessarily distinct. Note that a foreign key and the corresponding primary key should be defined on the same domain(s). Employee Dept Emp# e1 e2 e3 ename red blue brown Worksfordept d1 d2 Dept d1 d2 d3 Dname Pay Tax Art Foreign key

34. Referential Integrity If base relation R2 includes a foreign key FK matching the primary key PK of some base relation R1 then every value of FK in R2 must either (a) be equal to the value of PK in some tuple of R1, or (b) be wholly null. Note that PK and FK may comprise more than one attribute and that R1 and R2 are not necessarily distinct. ( Stated more simply a foreign key should be a valid primary key value or null.)

35. Foreign Key Rules For each foreign key three rules need to be answered: Can the foreign key accept nulls ? What should happen on an attempt to delete the target of a foreign key reference? What should happen on an attempt to update the target of a foreign key reference ? Employee Dept Dept d1 d2 d3 Dname Pay Tax Art Emp# e1 e2 e3 ename red blue brown Worksfordept d1 d2

36. Foreign Key Rules When should foreign key rules be checked ? Dept (Dept#, Dname, Budget) Emp (Emp#, Ename, Salary, WorksforDept#) WorksforDept# References Dept delete cascades, update cascades Depend (Emp#, Dependname, Date-of-birth) Emp# references Emp delete cascades, update cascades

37. Example of the Class table. Primary Key

38. Foreign key example

39. Relational instances in the Student relation • The content of the table (a grouping of rows) are called relational instances • These instances are unordered, and no two rows can be exactly alike A relational instance

40. Integrity Constraints • All DBMS’ must have some form of ICs to prevent invalid data from being entered. • Domain constraints specify the set of values which may be used for each field. • Other constraints, such as key or tuple, may limit which values from the domain can be used for a given field in a given instance. • Key constraints require that each set of fields in the key be unique for each entry.

41. Enforcing Integrity Constraints • Each DBMS should have means to resolve invlaid entries such as: • What happens if an entry that duplicates a key entry is entered? • What should be done if an entry of a foreign key is deleted? • A foreign key is a key where at least one field depends on a field from a different table. • What happens when an invalid entry is entered?

42. Relation schema • Associated with each attribute is a set of values, called a domain, that can be assigned to the entry of a tuple corresponding to the attribute. • A relation schema is a set of attributes. • Example EMP = { Name, SSN, DeptName, Salary, Birthdate } • Convention EMP(Name,SSN,DeptName,Salary,Birthdate)

43. Relational DBMS Products IBM Relational Products DB2 MVS/370 MVS/XA SQL/DS VM/CMS DOS/VSE QMF front-end to DB2 and SQL/DS CSP application development tool Numerous other RDBMS ORACLE (SQL*Forms) INGRES from ASK Corp. (OSL,ABF) AIM/RDB from Fujitsu INFORMIX VAXSQL/Rdb from DEC NonStop SQL from Tandem Microcomputer versions ORACLE INGRES dBase IV microSQL practically all micro DBMS

44. The relational model is based on set operations. Tables are sets of rows. The actual storage structure is hidden from the user. The relational model is just concerned with a logical view of the data, not the physical view. There are no pointers for the user to worry about. The only data are explicit values in tables. All data values in the cells of tables are Atomic (also known as Scalar). Exactly one data value and not a set or a repeating group is allowed in each cell. • Relational databases are the most widely used in the world ( 90+%.) • A mathematical viewpoint helped to shape a database industry. • Future? Possibly Object Oriented Database model.