Databases

• Entity:

  An entity is a person, place, thing, event or concept of interest to the business or organisation about

  which data is to be stored. For example, in a school, possible entities might be PUPIL, CLASS and

  TEACHER. An entity is implemented as a table.
• Entity occurrence:

  A specific example of an entity is called an instance or entity occurrence. For example, 001, John Smith,

  5/7/2010, 4F1 would be an example of an entity occurrence found in the PUPIL entity.
• Attribute:

  An entity is described by its attributes. Each attribute is a characteristic of the entity. For example, attributes of the PUPIL entity would include pupilID, forename, surname, DOB and regClass. An attribute is implemented as a field.
• Primary key:

  An attribute or combination of attributes that uniquely identifies one, and only one, entity occurrence is called a primary key. For example, the primary key of the PUPIL entity would be pupilID. A primary key is

  signified by underlining in the entity-relationship diagram. The primary key must be unique and not empty (entity integrity).
• Foreign key:

  A foreign key is an attribute in one table that uniquely identifies a row of another table. It is signified by an asterisk. The attribute refers to the Primary Key of another entity and used to link tables. A FK must

  already exist as a PK in another table and not be empty. (Referential Integrity)
• Compound key:

  A compound key is a primary key that comprises two or more attributes. Each attribute that makes up a

  compound key is a primary key in its own right. For example, the primary key of the CLASS entity would

  be the compound key formed by combining classID + staffID + subject. In this example, staffID would be

  the primary key of the TEACHER entity and subjectID would be the primary key of the SUBJECT entity.
• In some entities, no one attribute can be used as a primary key. One solution is to use a combination of two or more attributes to create a unique identifier. This is known as a compound key.
• A relationship is a natural association between one or more entities. For example, pupils learn subjects and teachers educate pupils.