How to Convert ER Diagram to Relational Database | Learn Databases
Entity-Relationship (E-R) Modeling: A method for designing databases Oracle to MS-Access and then using the Tool/Relationships capability of MS-Access to draw the One-to-many relationships are modeled by a foreign key attribute in the table First normal forms requires that there be no multi-valued attributes ( e.g. MS Access Database Tables SQL Reports Queries Plus One Commerce. . 19 Entities Attributes Identifiers Relationships Elements of ER Model 27 Derived attribute: The attribute derived from a stored attribute is called a. For example, a doctor performing drug tests should be able to access the patients ' medical data . ER diagram notation for composite attribute domain, name.
Although it is constructed in such a way as to allow easy translation to the relational schema model, this is not an entirely trivial process. The ER diagram represents the conceptual level of database design meanwhile the relational schema is the logical level for the database design. We will be following the simple rules: Entities and Simple Attributes: An entity type within ER diagram is turned into a table.
You may preferably keep the same name for the entity or give it a sensible name but avoid DBMS reserved words as well as avoid the use of special characters. Each attribute turns into a column attribute in the table. The key attribute of the entity is the primary key of the table which is usually underlined. It can be composite if required but can never be null. It is highly recommended that every table should start with its primary key attribute conventionally named as TablenameID.
Taking the following simple ER diagram: The initial relational schema is expressed in the following format writing the table names with the attributes list inside a parentheses as shown below for Persons personidname, lastname, email Persons and Phones are Tables.
Multi-Valued Attributes A multi-valued attribute is usually represented with a double-line oval.
If you have a multi-valued attribute, take the attribute and turn it into a new entity or table of its own. Then make a 1: N relationship between the new entity and the existing one.
MS Access Database Tables SQL Reports Queries Plus One Commerce. - ppt download
Create a table for the attribute. Add the primary id column of the parent entity as a foreign key within the new table as shown below: Persons personidname, lastname, email Phones phoneidpersonid, phone personid within the table Phones is a foreign key referring to the personid of Persons 3.
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.
How to Convert ER Diagram to Relational Database
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.
ER diagram notation for derived attribute, radius An attribute or set of attributes that uniquely identifies a particular entity is a key. However, to determine the class we need a composite key that consists of several attributes, such as catalogue number, section, semester, and year. In the ER diagram of Figure 7we underline the composite key, class.
The figure shows another attribute DragExpWeek of LabSection that stores the week of the semester in which the drag experiment occurs. Definition An attribute or set of attributes that uniquely identifies a particular entity is a key. A composite key is a key that is a composite of several attributes. For example, the student entity type is related to the team entity type because each student is a member of a team.
In this case, a relationship or relationship instance is an ordered pair of a specific student and the student's particular physics team, such as Emanuel Vagas, PhysFA04where PhysFA04 is Emanuel's team number.
Figure 8 illustrates three relationships. Unfortunately, Itnatios Trekas had to drop the course and retake it another semester. Consequently, his name is associated with two team numbers. We arrange the diagram so that the relationship reads from left to right, "a student is a member of a team.
Definition A relationship type is a set of associations among entity types.
- MS Access Database Tables SQL Reports Queries Plus One Commerce.
A relationship or relationship instance is an ordered pair consisting of particular related entities. The degree of a relationship type is the number of entity types that participate. Thus, the LabSecMemberOf relationship type of Figure 9 has degree 2, which we call a binary relationship type.