Constraints on Generalization
To model an enterprise more accurately, there are some constraints that are applicable onto the database on the particular generalization.
There are three types of constraints on generalization which are as follows:
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- First one determines which entity can be a member of the low-level entity set.
- Second relates to whether or not entities belong to more than one lower-level entity set.
- Third specifies whether an entity in the higher level entity set must belong to at least one of the lower level entity set within generalization.
1. First one determines which entity can be a member of the low-level entity set:
Such kind of membership may be one of the following:
- Condition-defined –
In this lower-level entity sets, evaluation of membership is on basis whether an entity satisfies an explicit condition or not. For example, Let us assume, higher-level entity set student which has attribute student type. All the entities of student are evaluated by definition of attribute of student. Entities are accepted by the satisfaction of condition i.e. student type = “graduate” then only they are allowed to belong to lower-level entity set i.e. graduate student. By the satisfaction of condition student type = “undergraduate” then they are included in undergraduate student. In fact, all the lower-level entities are evaluated on the basis of the same attribute, thus it is also referred as attribute-defined.
- User-defined –
In this lower-level entity sets are not get constrained by a condition named membership; users of database assigns entities to a given entity set. For example, Consider a situation where after 3 months of employment, the employees of the university are assigned to one of four work teams. For this purpose, we represent teams them as four lower-level entity sets of higher-level employee entity set. On the basis of an explicit defining condition, a given employee is not assigned to specific team entity. User in charge of this decision makes the team assignment on an individual basis. By adding entity to an entity set, assignment is implemented.
2. Second relates to whether or not entities belong to more than one lower-level entity set :
Following is one of the lower-level entity sets:
- Disjoint –
The requirement of this constraint is that an entity should not belong to no more than one lower-level entity set. For example, the entity of student entity satisfy only one condition for student type attribute i.e. Either an entity can be a graduate or an undergraduate student, but cannot be both at the same time.
- Overlapping –
In this category of generalizations, within a single generalization, the same entity may belong to more than one lower-level entity set. For example, in the employee work-team assume that certain employees participate in more than one work team. Thus, it offers a given employee that he may appear in more than one of the team entity sets that are lower-level entity sets of employee. Thus, generalization is overlapping.
3. Third specifies whether pr not an entity in the higher level entity set must belong to at least one of the lower level entity set within generalization :
This constraint may be one of the following:
- Total generalization or specialization –
According to this constraint, each higher-level entity must belong to a lower-level entity set.
- Partial generalization or specialization –
According to this constraint, some higher-level entities may not belong to any lower-level entity set.