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74 Chapter 3 Data Modeling Using the Entity-Relationship (ER) Model Figure 3.12 A 1:1 relationship, MANAGES. 3.4.3 Constraints on Relationship Types Relationship types usually have certain constraints that limit the possible combinations of entities that rnay participate in the corresponding reiationship set. These constraints are deternrined from the miniworld situation that the relationships represent. For exanrple, in Figure 3.9, if the contpany has a rule that each employee must work for exactll, one department, then we would like to describe <strong>this</strong> constraint in the schema. We can distinguish two main types of relationship constraints: cardinality rotio and porticipation. Cardinality Ratios for Binary Relationships. The cardinality ratio for a binary relationship specifies the rnaxinuri number of relationship instances that an entity can participate in. For example, in the WORKS_FOR binary relationship type, DEPARTMENT:EMPLOYEE is of cardinality ratio 1:N, meaning that each department can be related to (that is, eniploys) any nun"rber of employees,e but an employee can be related to (work for) only one department. The possible cardinality ratios for binary relationship types are 1:i, 1:N, N:1, and M:N. An examprle of a l:l binary relationship is MANAGES (Figure 3.12), which relares a department entity to the employee who manages that department. This represents the minirvorld constraints that-at any point in time-an ernployee can manage one departrnent only and a department can have one manager only. The relationship type WORKS_ON (Figure 3.13) is of cardinality ratio M:N because the niniworld rule is that an employee can work on several projects and a project can have several employees. EMPLOYEE MANAGES DEPARTMENT 9, N stands Ior any number of related entitles (zero or more), d1 d2 ds
nbina- These _rs repployee s con- ,1 COn- binary entity ) type, -tment 'ee can ios for rlates a :esents lanage lationminin have '3u EMPLOYEE €1 e2 €a 3.4 Relationship Types, Relationship Sets, Roles, and Structural Constraints WORKS ON PROJECT t-ardinality ratios for binary relationships are represented on ER diagrams by displaving l, M, and N on the diamonds as shown in Figure 3.2. Participation Constraints and Existence Dependencies. The participation constraint specifies whether the exister.rce of an entity depends on its being reiated ttr irnother entity via the relationship type. This constraint specifies the minimurn number of relationship instances that each entity can participate in, and is somet imes called the minimum cardinality constraint. There are two types of participation constraints-total and partial-which we illustrate by example. If a company policy states thal every employee must work for a department, then an employee r.rltity can exist only if it participates in at least one WORKS FOR relationship instance (Figure 3.9). Thus, the participation of EMPLOYEE in WORKS_FOR is c.rlled total participation, meaning that every entity in the total sel of employee cntities must be related to a department entity via WORKS_FOR. Total participation is trlso called existence dependenry. In Figure 3.12 we do not expect every employee to nanage a department, so the participation of EMPLOYEE in the MANAGES reiationship type is partial, meaning thar some or part of the set o/employee entities are rc'lated to some department entity via MANAGES, but not necessarily all. We will ret-er to the cardinality ratio and participation constraints, taken together, as the structural constraints of a relationship type. Pt Pz Ps P4 Figure 3.13 An M:N relationship, WORKS_ON
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