CSci Database Systems - Chapter 7
The entity relationship (ER) data model has existed for over 35 years. This database contains information about employees, departments and projects. Jan 26, To design the conceptual schema for a database application. Overview of Examples: an EMPLOYEE entity may have the attributes. Name. Translation of a (Normal Form) ER Diagram to a RDB. ❑ A Normal Form for ER .. E.g. In an EMPLOYEE database, we want to describe information about employee .. Arrow of a relationship type show the functional dependency. E.g. Course.
Person[Role, Name, Age, Salary] Attributes may still have key uniqueness constraints, making them a key attribute that uniquely identifies its parent entity. Entities may have no key attributes making them weak entities. Composite keys must be combined into a composite attribute. There may be multiple keys, but none of them is a primary key.
- Data Modeling Using the Entity-Relationship (ER) Model
- Employee management system ( Entity Relationship Diagram)
Each department has a unique name, a unique number, and a particular employee who manages the department. We keep track of the start date when that employee began managing the department.
A department may have several locations. A department controls a number of projects, each of which has a unique name, a unique number, and a single location.
An employee is assigned to one department, but may work on several projects, which are not necessarily controlled by the same department. We keep track of the current number of hours per week that an employee works on each project.
We also keep track of the direct supervisor of each employee who is another employee. We want to keep track of the dependents of each employee for insurance purposes. Some attributes here hint at relationships… Dependent has Employee as an attribute, but couldn't Employee have a multivalued composite Dependent attribute?
N relationships Here we must create a table R of tuples including the key of S skthe key of T tkand any attributes of R; we can not push the data into either S or T.
Employee management system | Editable Entity Relationship Diagram Template on Creately
The sk column of R should have a foreign key constraint referring to the key column of S, and the tk column of R should similarly have a foreign key constraint to the key column of T. Again, we would have an appropriate foreign key constraint back to the original table.
Make this a foreign key in E1. Make this new column a foreign key in E2, referencing E1.
For multivalued attributes of entity E, create a new relation R. One column of R will be E. Joins arise in steps 2, 3, 4, 5, 6, and 7, for recovering the original relationships or attribute sets for 6, or entities for 2. In every case, the join field is a key of one relation and a foreign key in the other. Not all joins are about recovering relations from an ER diagram.
Also, I said earlier that entity T should not have an attribute that was another entity of type S; instead, we should create a relationship R between T and S. If S was at all a candidate for an attribute, each T would be related to at most one S and so this would have cardinality constraint TNRS.
Then, when we did the above conversion, in step four we would add S's key to T with a foreign key constraint referring to S.
DATA MODELING USING THE ENTITY-RELATIONSHIP MODEL
But suppose we did add S as an entity attribute to T. Then we would end up with the same situation: So in the end we get the same thing. Invoice How shall we model invoices? An invoice is a collection of parts ordered, each with a quantity. An invoice is thus all the items to the same customer with the same date.
For a given c and d there might be multiple parts p that were part of the invoice. Ternary relationships tend to be inefficient. Even if we do this, we have another issue: That is, we declare that orders are "things" rather than relationships.
This is an instance of a rather general strategy that might be called the synthetic-key trick: In this case the synthetic key has a natural interpretation: