To date we have a Student Record System which will record students details and car details. This is shown in figure 8.1.

 

Entity Relational diagram showing STUDREC in a One to many Join with CARS

Figure 8.1: Students can own many cars

Figure 8.1 needs some explanation. The terms used are common in the field of Systems Analysis and in particular SSADM these topics are properly covered in detail in Systems Analysis course.

Systems Analysis:
The work done in studying a problem to decide how best it can be handled by a computer. It can cover a wide area, e.g. the way a company works, how data moves from one part of the company to another, the design of forms and reports etc.

Longman Illustrated Dictionary of Computer Science - p 191.

 
 
SSADM
Structured Systems Analysis & Design Method. Launched in 1981 as a bid to standardise IT projects across UK government departments.

This rounded box represents an entity. An entity is a thing about which our database system holds information. It has the potential to occur more than once and has a number of data items associated with it to describe each occurrence.

e.g. Cars can occur many times in the CARS table. They are identified by REGNO and have other data items associated with them (COLOUR, MAKE etc.)

 

 

 

Entity: (rounded rectangle)

Figure 8.2: Entity

 

 

 

 

 

 

 

 

 

A relationship is a logical connection between two entities, e.g. a student can own a car, or even two or three cars. This is known as a one to many relation and is depicted by a "crows foot" symbol (Figure 8.3).

 

 

Relationship: this is a line with a three pronged "crows foot" at one end. The symbol depicts a one to many relationship.

Figure 8.3: "Crows Foot"

 

 

 

 

 

 

The student/cars relationship may be depicted as shown in Figure 8.4.

 

Diagram showing one entity (STUDENTS (studrec) joined to cars in a one to many Join)

Figure 8.4: Logical Data Structure of Students and Cars

 

This is known as a Logical Data Structure, it models the relationships between entities.