An Entity Relationship Diagram, otherwise known as an ER Diagram, is a graphic used for demonstrating how one entity in a database is related to another. There are three types of ER Diagrams. The first is one-to-one, where one entity is linked to one other. The second is one-to-many, where one entity is linked to more than one entity. The third is many-to-many where more than one entity is linked to more than one other entity. Entities are represented in rectangular boxes, relationships are represented in diamond boxes and attributes are represented in oval boxes. Entities and their attributes are linked by a straight line between the two and entities are linked to each other by a straight line bisected by their relationship. For instance, an ER Diagram about teenagers and smartphones would have the words ‘teenager’ and ‘smartphone’ in rectangular boxes with a line coming out of each, linking them to the word ‘uses’ in a diamond box because teenagers use smartphones. In oval boxes linked by straight lines to the ‘teenager’ box would be words like ‘name’, ‘age’ and ‘lives’. In oval boxes linked by straight lines to the ‘smartphone’ box would be words like ‘brand’, ‘price’ and ‘features’. A one-to-many diagram could have ‘close friends’ in a rectangular box linked to ‘teenager’ by a straight line bisected by ‘has’ in a diamond box. When the data has been collected, these words will be replaced by the data. The benefit of an ER Diagram is that it can clearly and simply show relationships between two or more entities so links and conclusions can be more easily brought. The standardised box shapes also help in identifying what are entities, what are relationships and what are attributes because in a many-to-many ER Diagram the words could easily get mixed up.
