ERDs What Design and implementation do they hold?

1. ERDsWhat Design and implementation do they hold?

2. What info can we get from ERDs? Minimum and Maximum cardinality is a rich source of information. It tells you how many of each entity in a relationship can participate. It also says whether or not an entity has to exist (minimum cardinality) The relationship lines help to identify attributes in entities since (in general) the primary key of one entity will join to a foreign key in other entities which is the same field-type There is a difference between ERDs which represent a logical view of the system and the physical view of the system. The logical view may have many-to-many relationships whereas the physical view will not. One-to-Many relationships are often characterised as Master-Detail or Header-Items, this reflects their use in modelling trade documents, such as a Sales Orders, which have one header part and many order item lines One-to-Many relationships can sometimes be characterised as Master-Transactional, that is to say that the many table is a transactional table which has records added to it frequently and consists mainly of ID fields and numeric fields, whereas the one table is a master table which changes infrequently and has more descriptive fields