Connectivity

Connectivity refers to the number of entity instances. An entity instance is a particular occurrence of an entity. The following figure shows that the three types of connectivity are one-to-one (written 1:1), one-to-many (written 1:n), and many-to-many (written m:n).

Figure 1. Connectivity in relationships

Diagram shows an example for each of the three types of relationship. The one-to-one relationship shown has two rectangles with a single line connecting them. The one-to-many relationship shown has one rectangle above four other rectangles that are in a horizontal row. There is one line connecting each of the lower rectangles to the single upper rectangle. The many-to-many relationship shows two rows of rectangles. The upper row has three and the lower row has four. Each rectangle is connected to a point in between the two rows by a single line. This is to show that each of the upper rectangles is connected to all of the lower rectangles and vice versa.

For instance, in the telephone directory example, an address can be associated with more than one name. The connectivity for the relationship between the address and name entities is one-to-many (1:n).