An entity relationship (ER) diagram is a documentation technique used to represent the relationship between entities. Entities are physical resources (automobiles, cash, or in-ventory), events (ordering inventory, receiving cash, shipping goods), and agents (sales-person, customer, or vendor) about which the organization wishes to capture data. One common use for ER diagrams is to model an organization’s database, which we examine in detail in Chapter 9. Figure 2-14 shows the symbol set used in an ER diagram. The square symbol represents entities in the system.
The labeled connecting line represents the nature of the relationship between two entities. The degree of the relationship, called cardinality, is the numerical mapping between entity instances. A relationship can be one-to-one (1:1), one-to-many (1:M), or many-to-many (M:M). 2 If we think of entities in the ER diagram as files of records, cardinality is the maximum number of records in one file that are related to a single record in the other file and vice versa. Cardinality reflects normal business rules as well as organizational policy. For instance, the 1:1 cardinality in the first example in Figure 2-14 suggests that each salesperson in the organization is assigned one automobile.
If instead the organization’s policy were to assign a single automobile to one or more salespersons who share it, this policy would be reflected by a 1:M relationship. Similarly, the M:M relationship between vendor and inventory in Figure 2-14 implies that the organization buys the same type of products from one or more vendors. A company policy to buy particular items from a single vendor would be reflected by a 1:M cardinality. System designers identify entities and prepare a model of them, similar to the one presented in Figure 2-15. This data model is the blueprint for what ultimately willa