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ADDITIONAL FEATURES OF THE ER MODEL
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We now look at some of the constructs in the ER model that allow us to describe some subtle properties of the data The expressiveness of the ER model is a big reason for its widespread use
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241 Key Constraints
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Consider the Works In relationship shown in Figure 22 An employee can work in several departments, and a department can have several employees, as illustrated in the Works In instance shown in Figure 23 Employee 231-31-5368 has worked in Department 51 since 3/3/93 and in Department 56 since 2/2/92 Department 51 has two employees Now consider another relationship set called Manages between the Employees and Departments entity sets such that each department has at most one manager, although a single employee is allowed to manage more than one department The restriction that each department has at most one manager is an example of a key constraint, and it implies that each Departments entity appears in at most one Manages relationship
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The Entity-Relationship Model
in any allowable instance of Manages This restriction is indicated in the ER diagram of Figure 26 by using an arrow from Departments to Manages Intuitively, the arrow states that given a Departments entity, we can uniquely determine the Manages relationship in which it appears
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Employees
Manages
Departments
Key Constraint on Manages
An instance of the Manages relationship set is shown in Figure 27 While this is also a potential instance for the Works In relationship set, the instance of Works In shown in Figure 23 violates the key constraint on Manages
123-22-3666 231-31-5368
3/3/93 51 2/2/92 56 60
131-24-3650 223-32-6316 3/1/92
EMPLOYEES Partial participation
MANAGES One to Many
DEPARTMENTS Total participation
An Instance of the Manages Relationship Set
A relationship set like Manages is sometimes said to be one-to-many, to indicate that one employee can be associated with many departments (in the capacity of a manager), whereas each department can be associated with at most one employee as its manager In contrast, the Works In relationship set, in which an employee is allowed to work in several departments and a department is allowed to have several employees, is said to be many-to-many
2
If we add the restriction that each employee can manage at most one department to the Manages relationship set, which would be indicated by adding an arrow from Employees to Manages in Figure 26, we have a one-to-one relationship set
Key Constraints for Ternary Relationships
We can extend this convention and the underlying key constraint concept to relationship sets involving three or more entity sets: If an entity set E has a key constraint in a relationship set R, each entity in an instance of E appears in at most one relationship in (a corresponding instance of) R To indicate a key constraint on entity set E in relationship set R, we draw an arrow from E to R In Figure 28, we show a ternary relationship with key constraints Each employee works in at most one department, and at a single location An instance of the Works In3 relationship set is shown in Figure 29 Notice that each department can be associated with several employees and locations, and each location can be associated with several departments and employees; however, each employee is associated with a single department and location
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A Ternary Relationship Set with Key Constraints
242 Participation Constraints
The key constraint on Manages tells us that a department has at most one manager A natural question to ask is whether every department has a manager Let us say that every department is required to have a manager This requirement is an example of a participation constraint; the participation of the entity set Departments in the relationship set Manages is said to be total A participation that is not total is said to be partial As an example, the participation of the entity set Employees in Manages is partial, since not every employee gets to manage a department
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