An entity relationship ( ER ) diagram is a certification technique used to stand for the relationship between entities. Entities are physical resources ( cars. hard currency. or in-ventory ) . events ( telling stock list. having hard currency. transporting goods ) . and agents ( sales-person. client. or seller ) about which the organisation wishes to capture informations. One common usage for ER diagrams is to pattern an organization’s database. which we examine in item in Chapter 9. Figure 2-14 shows the symbol set used in an ER diagram. The square symbol represents entities in the system.
The labelled connecting line represents the nature of the relationship between two entities. The grade of the relationship. called cardinality. is the numerical function between entity cases. A relationship can be one-to-one ( 1:1 ) . one-to-many ( 1: Meter ) . or many-to-many ( M: M ) . 2 If we think of entities in the ER diagram as files of records. cardinality is the maximal figure of records in one file that are related to a individual record in the other file and frailty versa. Cardinality reflects normal concern regulations every bit good as organisational policy. For case. the 1:1 cardinality in the first illustration in Figure 2-14 suggests that each sales representative in the organisation is assigned one car.
If alternatively the organization’s policy were to delegate a individual car to one or more sales representatives who portion it. this policy would be reflected by a 1: Meter relationship. Similarly. the Meter: M relationship between seller and stock list in Figure 2-14 implies that the organisation buys the same type of merchandises from one or more sellers. A company policy to purchase peculiar points from a individual seller would be reflected by a 1: M cardinality. System interior decorators identify entities and fix a theoretical account of them. similar to the 1 presented in Figure 2-15. This information theoretical account is the design for what finally willa