Chapter 5 The Relational Database Model: Introduction Fundamentals of Database Management Systems, 2nd ed. by Mark L. Gillenson, Ph.D. University of Memphis John Wiley & Sons, Inc.

Chapter Objectives Explain why the relational database model became practical in about 1980. Define such basic relational database terms as relation and tuple. Describe the major types of keys including primary, candidate, and foreign.

Chapter Objectives Describe how one-to-one, one-to-many, and many-to-many binary relationships are implemented in a relational database. Describe how relational data retrieval is accomplished in concept with the select, project, and join operators. Understand how the join operator facilitates data integration in relational database.

Relational Database Model In 1970, E. F. Codd published “A Relational Model of Data for Large Shared Data Banks” in CACM. In the early 1980s, commercially viable relational database management systems became available.

Relational Database Model While relational database was very tempting in concept in the 1970s, it was not easily applicable in a real-world environment for reasons related to performance. The earlier hierarchical and network database management systems were just coming onto the commercial scene and were the focus of intense marketing efforts by the software and hardware vendors.

The Relational Database Concept Data appears to be stored in what we have been referring to as simple, linear files. Relational databases are based on mathematics. A relational database is a collection of relations that, as a group, contain the data that describes a particular business environment.

Relational Terminology Relations - what we have been referring to as simple linear files. Also called tables. Row = record (files) = tuple (relation) Column = field (files) = attribute (relation)

Relational Database Terminology

File / Relation: Differences The columns of a relation can be arranged in any order without affecting the meaning of the data. This is not true of a file. The rows of a relation can be arranged in any order, which is not true of a file.

File / Relation: Differences Every row/column position (a cell) can have only a single value, which is not necessarily true in a file. No two rows of a relation are identical, which is not necessarily true in a file.

Primary Key A relation always has a unique primary key. A primary key (also called “the key”) is an attribute or a group of attributes whose values are unique throughout all of the rows of the relation.

Primary Key

Primary Key The number of attributes involved in the primary key is always the minimum number of attributes that provide the uniqueness quality. In the worst case, all of the relation’s attributes combined could serve as the primary key.

Candidate Key If a relation has more than one attribute or minimum group of attributes that represents a way of uniquely identifying the entities, then they are each called a candidate key. When there is more than one candidate key, one of them must be chosen to be the primary key of the relation.

Candidate Key Which candidate key to pick depends on the application using the database. Alternate key is a candidate key that was not chosen to be the primary key of the relation.

Foreign Key An attribute or group of attributes that serves as the primary key of one relation and also appears in another relation (foreign key in this relation).

Foreign Key Crucial in relational database, because the foreign key is the mechanism that ties relations together to represent unary, binary, and ternary relationships. Foreign key attribute must have same domain of values as Primary key attribute in other relation.

Domain of Values Two attributes have the same domain of values if the attributes have values of the same type. e.g., Salesperson Number in SALESPERSON and in CUSTOMER - three digit whole numbers that are the identifiers for salespersons.

Binary Relationships One-to-One One-to-Many Many-to-Many

One-to-Many Binary Relationship Salesperson Customer The Salesperson Number foreign key in the CUSTOMER relation effectively establishes the one-to-many relationship between salespersons and customers.

Foreign Key Can Be A Part of The Primary Key Customer Customer Employee

General Hardware Co.

Many-to-Many Binary Relationship Salesperson Product

Many-to-Many Relationship

Intersection Data

Many-to-Many Relationship Has its own relation in the database. Can have its own attributes. It is a kind of entity -- an Associative Entity

SALES Relation (modified) A Date attribute is required if the data may be stored two or more times in a year. A Time attribute is required if the data may be stored more than once in a day.

Unacceptable: Many-to-Many

SALES Relation (without intersection data)

One-to-One Binary Relationship

General Hardware Co. including OFFICE

General Hardware Co. including OFFICE Can SALESPERSON and OFFICE be combined into one relation?

Data Retrieval from a Relational Database The discussion thus far has concentrated on: how a relational database is structured loading a database with data Let’s discuss the effort to retrieve the data in a way that is helpful and beneficial to the business organization that built the database.

Relational DBMS Have the ability to accept high level data retrieval commands Process the commands against the database’s relations and return the desired data.

The Relational Select Operator Retrieves a horizontal slice of the relation. Select rows from the SALESPERSON relation in which Salesperson Number = 204. The result of a relational operation will always be a relation.

The Relational Project Operator Retrieves a vertical slice of the relation. Project the Salesperson Number and Salesperson Name over the SALESPERSON relation.

Extracting Data Across Multiple Relations: Data Integration A DBMS must be able to store data nonredundantly while also providing a data integration facility. Relational DBMSs automate the cross-relation data extraction process in such a way that it appears that the data in the relations is integrated while also remaining nonredundant.

Data Integration The relational algebra Join command. Join the SALESPERSON relation and the CUSTOMER relation, using the Salesperson Number of each as the join fields. Select rows from that result in which Customer Number = 1525. Project the Salesperson Name over that last result.

Terminology Cartesian Product - comparing every possible combination of two sets, or two relations. Equijoin - a join where two join field values are identical. Natural join - one of the two identical join columns is eliminated.

Good Reading Bookstores

World Music Association

Lucky Rent-A-Car

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