connects more than two entity sets. ◇Suppose that drinkers will only drink certain beers at certain bars. ◇ Our three binary relationships Likes, Sells. ✓entity or a relationship? ✓Identifying relationships: ✓Binary or ternary? Aggregation? ▫ Note constraints of the ER Model: ✓A lot of data semantics can ( and. DATA MODELING USING THE ENTITY-RELATIONSHIP MODEL . relationship type SUPPLY; (c) another example of ternary versus binary relationship type.
Journalist writes an article. This example can be implemented very easily. In the diagram below, we represent our ternary relationship with an extra table, which can be modelled in Vertabelo very quickly.
N-ary relationship types
In other words, a group can have specific classess only at one classrom. Sometimes it is possible to replace a ternary or n-ary relationship by a collection of binary relationship connecting pairs of the original entities. However, in many cases it is hard to replace ternary relationship with two or more binary relationships because some information could be lost. Another ternary relationship presents a different situation — Teacher recommends a book for a class: In the example with groups and classes, the primary key consisted only of two foreign keys.
This meant that there could be only one classroom for a specific group and class. In this situation the primary key consists of all three foreign keys. It makes a significant difference, because in this case it is possible to recommend more than one book for a specific course by one teacher. It is possible that the teacher recommended the book for another class. We can tell who recommends the book and which book is used in which class.
To simplify the analysis, we assume that the database only stores results related to this experiment over a period of several years. Entity An entity is a real-world item or concept that exists on its own.
In our example, a particular student such as, "Emanuel Vagas"team, lab section, or experiment is an entity. The set of all possible values for an entity, such as all possible students, is the entity type.
ER Diagram Representation
In an ER model, we diagram an entity type as a rectangle containing the type name, such as student see Figure 2.
Definition An entity is a real-world item or concept that exists on its own. The set of all possible values for an entity is the entity type. ER diagram notation for entity student Attribute Each entity has attributes, or particular properties that describe the entity. For example, student Emanuel Vagas has properties of his own Student Identification number, name, and grade.
A particular value of an attribute, such as 93 for the grade, is a value of the attribute. Most of the data in a database consists of values of attributes.
The set of all possible values of an attribute, such as integers from 0 to for a grade, is the attribute domain. In an ER model, an attribute name appears in an oval that has a line to the corresponding entity box, such as in Figure 3.
Definition An attribute of an entity is a particular property that describes the entity. The set of all possible values of an attribute is the attribute domain.
Sometimes the value of an attribute is unknown or missing, and sometimes a value is not applicable. In such cases, the attribute can have the special value of null. For example, until the professor grades a laboratory assignment, the team grade is missing or null. Definition Null is the special attribute value that indicates an unknown or missing value. An attribute can be simple or composite. A simple attribute, such as grade, is one component that is atomic. If we consider the name in two parts, last name and first name, then the name attribute is a composite.
A composite attribute, such as "Emanuel Vagas", has multiple components, such as "Emanuel" and "Vagas"; and each component is atomic or composite. We illustrate this composite nature in the ER model by branching off the component attributes, such as in Figure 4.
Definition A simple attribute is one component that is atomic. A composite attribute has multiple components, each of which is atomic or composite. ER diagram notation for composite attribute domain, name Another way to classify attributes is either as single-valued or multi-valued.
For an entity an attribute, such as StudentGrade, usually holds exactly one value, such as 93, and thus is a single-valued attribute. However, two lab assistants might assist in a laboratory section. Consequently, the LabAssistant attribute for the entity LabSection is multi-valued. A multi-valued attribute has more than one value for a particular entity.
We illustrate this situation with a double oval around the lab assistant type, LabAssistant see Figure 5. Definition For a particular entity, an entity attribute that holds exactly one value is a single-valued attribute.
ER diagram notation for multi-valued attribute domain, LabAssistant A derived attribute can be obtained from other attributes or related entities. For example, the radius of a sphere can be determined from the circumference.
We request the derived attribute with a dotted oval and line, such as in Figure 6.