Entity Relationship Diagram
A data is a set of concepts that can be used to describe the structure of a database. By structure of a database, we mean data types, relationships and constraints.
A systems analyst gathers the requirements from stakeholders using various elicitation techniques in the analysis phase. The requirements are data requirements and functional requirements which are used to describe the myriad transactions of the system.
The data requirements are used to construct a conceptual data base design which is a concise description of the data requirements given by the user. They include a detailed description of data types, relationships and constraints. The conceptual schema/data base design is independent of implementation.
The ER model describes data as entities, relationships and attributes.
(a) Entities and attributes
1. Entities: an entity is a thing which exists in the world with an independent existence. An entity may have a physical existence (for example, a person, a car or a house) or it may have a conceptual existence (for example, a company, a job, a course, an order or an invoice).
2. Attributes: each entity has a set of properties called attributes that describe the entity.
An entity will have a value for each of its attributes. Attributes can be categorized in the following ways:
(i) Composite Vs Atomic attributes: atomic attributes cannot be broken down further while composite attributes can be broken down further. (for example, a name attribute can be broken down into the first name, middle name and last name, whereas the age attribute cannot be broken further down.)
(ii) Single value Vs multi value attribute: an entities attributes may have a single value or multiple values. The entity car has as its attributes as engine number and color. For each car there will be only one engine number, while a single car may have more than one colors.
(iii) Stored Vs derived: some values are stored in the system which other may need to be derived through calculations based on the values of other stored attribute values. For example, the rate and quantity of a commodity or product are usually stored values, while the total amount payable is derived as per a calculation of rate x quantity.
(b) Entity type, value set and attribute: an entity type defines a set of entities that have the same attributes. For example, a trader will probably have many customers. He will like to store the information about his customers so that he is able to offer them better services. The customer entity will have same attributes for all the customers, but each will have different values.
1. Value set/Domain of attributes: each attribute of any entity type is associated with a value set or domain specifies the set of values that may be assigned to that attribute for each individual entity. For example, Bill entity type will have the bill number as an attribute whose value set is let's say, 1 to 99999, it means that each time a bill number is assigned, it must be between the above range defined. The range defined is the value set for the bill number.
2. Key attributes of an entity type: an important constraint on entities of the entity type is a key or uniqueness constraint on the attribute. Values of these attributes are unique on the attribute. Values of these attributes are unique or distinct for each entity in entity set and such an attribute is called its key attribute. Key attributes can be formed by a single attribute or a combination of many attributes.
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