They specify the only values an attribute can have. In effect, a domain hierarchy is created with the data type at the highest level and acceptable values at the lowest. The ER approach to logical data modeling does not end here. What I have set forth in this article is just a cursory view of some rather complex notions involved in data modeling. Figure 8 shows how the topics I discuss in this article relate to each other.
Proper procedure is to label all relationships in both directions, as shown in the top figure.
However, many modelers place the relationship in a diamond, as shown in the bottom figure. Some modelers place attributes in the entity box, as shown in the top figure, while others place them in ovals, as shown in the bottom figure. Note that Chen and Reiner use a diamond to represent a relationship, while the Trident approach uses a line. The bar represents mandatory, while the oval represents optional. An entity type is related to itself.