Relational Database Theory. Relational Database Theory Database design using MySQL 21-Jan-06 : [1]

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1 Relational Database Theory Relational Database Theory Database design using MySQL 21-Jan-06 : [1]

2 Reminder MySQL is a Relational DBMS, so this section looks at Relational DB theory. The main reasons for using a database are to: reduce data redundancy (i.e. data duplication) achieve data independence (data storage independent of user applications) enforce data integrity (i.e. accuracy). Relational Database Theory Database design using MySQL 21-Jan-06 : [2]

3 Real World Database A database is a simplified representation or model of a real world situation. Probably the most popular method for modelling real world situations for relational databases is the: Entity-Attribute-Relationship (EAR) Model Relational Database Theory Database design using MySQL 21-Jan-06 : [3]

4 EAR Modelling Stems from perception of the real world as finite objects. Attributes describe entities which in turn are linked by relationships. Entity: a physical or abstract object that exists and can be distinguished from other objects. Attribute: a named characteristic of an entity. Relationship: a mapping between two entity classes. Relational Database Theory Database design using MySQL 21-Jan-06 : [4]

5 Relationships A direct mapping between two entity classes. one-to-one: an element from one class is mapped to exactly one element from another class. one-to-many: an element from one class is mapped to one or more elements in another class. many-to-many: zero, one or more elements from one class are mapped to zero, one or more elements in another class. Relational Database Theory Database design using MySQL 21-Jan-06 : [5]

6 EAR Example Students on a particular course each have a personal tutor for the duration of their study. During the course they undertake various modules, and an individual project. Relational Database Theory Database design using MySQL 21-Jan-06 : [6]

7 EAR Diagram TUTOR STUDENT PROJECT MODULE one-to-one relationship one-to-many relationship many-to-many relationship Relational Database Theory Database design using MySQL 21-Jan-06 : [7]

8 Keys An entity key is an single or set of attribute(s) that uniquely defines that entity. name address gender county number of residents hair colour dob Person NI no. weight height road name Building postcode house name or number Relational Database Theory Database design using MySQL 21-Jan-06 : [8]

9 What is there is no key? If there is no suitable unique key, can add an integer number key yourself. e.g. name gender person id number Person weight Relational Database Theory Database design using MySQL 21-Jan-06 : [9]

10 And, back to tables? So we ve gone from real world situation to EAR model diagram. Now what? A relational database consists of tables, records and fields. How does it all tie in to the EAR model? Relational Database Theory Database design using MySQL 21-Jan-06 : [10]

11 Tabulating room number name name year of study TUTOR STUDENT dob PROJECT MODULE description title title course code Relational Database Theory Database design using MySQL 21-Jan-06 : [11]

12 One Table? Student Name John Smith John Smith Jenny Crinion Student Yr of Study Student DOB Student Tutor Name Paul Tinker Paul Tinker John Foss Student Tutor Rm Project Title Aerosols Aerosols Hydraulics Project Description blah blah different blah Project Tutor John Foss John Foss Paul Tinker Project Tutor Rm Module Code Module Title database 1 database 2 database 2 Module Tutor Emily Fahye Paul Tinker Paul Tniker Module Tutor Rm Relational Database Theory Database design using MySQL 21-Jan-06 : [12]

13 Design from the EAR room number name name year of study TUTOR STUDENT dob PROJECT MODULE description title each entity has its own table one-to-one and one-to-many relationships can be included by adding an entity key into the relevant many -end table. many-to-many relationships will require their own table consisting of related entity keys. Relational Database Theory Database design using MySQL 21-Jan-06 : [13] title course code

14 Lots of tables? Table tutors tutor_id name Rm 1 Paul Tinker John Foss Emily Fahye 345 Table modules code title tutor_id O1 database 1 3 O2 database 2 1 Table enrolment student_id mod_code Table students student_id name tutor_id dob 1 John Smith 2 Jenny Crinion Table projects title description tutor_id student_id Aerosols blah 2 1 Hydraulics different blah 1 2 Relational Database Theory Database design using MySQL 21-Jan-06 : [14]

15 Keys The unique record keys in each of the tables are called primary keys. If this primary key consists of more than one field it is called a composite key. A key in a table that refers to the primary key in another table is called a foreign key. Relational Database Theory Database design using MySQL 21-Jan-06 : [15]

16 Primary and Foreign Keys tutor_id name Rm primary keys Table tutors 1 Paul Tinker John Foss Emily Fahye 345 code title tutor_id O1 database 1 3 O2 database 2 1 Table modules foreign keys Every primary key in a particular table must be unique. To ensure referential integrity all foreign keys must have a associated value as a primary key in the related table. Relational Database Theory Database design using MySQL 21-Jan-06 : [16]

17 Normalisation This process of decomposing data into lots of smaller tables which are related through the use of keys is called normalisation. It is essential to eliminate data redundancy (i.e. repeated data). Relational Database Theory Database design using MySQL 21-Jan-06 : [17]

18 Normalisation: Next step To ensure a fully normalised solution with no repeated data, it is also important not to store data that can be derived from elsewhere Table bank_accn Name Balance Overdraft Limit Available John Alice NO! Available = Balance + Overdraft Limit Relational Database Theory Database design using MySQL 21-Jan-06 : [18]

19 Data Integrity or Accuracy type checks e.g. ensure numeric field is numeric and not a string. redundancy checks e.g. check that a new record is not a copy of another. i.e. MySQL won t check your data very thoroughly. That the job of the input application. Relational Database Theory Database design using MySQL 21-Jan-06 : [19]

20 Data Integrity By Normalisation Sometimes it is desirable to restrict the data in a column to a set of values. Can either: A. Use an ENUM( value1, value2, ) field type definition. B. Use an addition table to store the values to choose from and relate the tables using primary and foreign keys. Relational Database Theory Database design using MySQL 21-Jan-06 : [20]

21 ENUM vs. Normalisation Table person Name Gender Hair_id John male 1 Alice female 3 James male 1 name Person gender hair colour Table hair Hair_id Colour 1 Blonde 2 Brown 3 Black Why hair_id? Performance is better with integer keys. Also, if we join on a ID key, we can change the names of the hair colours without touching any other tables Relational Database Theory Database design using MySQL 21-Jan-06 : [21]

22 Review Familiar with the concept of modelling the real world using the EAR method constructing an EAR diagram mapping this diagram to a relational database schema the terms normalisation, primary and foreign keys. data integrity and some of the measures that can be taken to enforce it. Relational Database Theory Database design using MySQL 21-Jan-06 : [22]

23 Group Case Study HOE 8 Designing a Database Relational Database Theory Database design using MySQL 21-Jan-06 : [23]