As a result of this separation of concerns, mappings are needed to allow
navigation between the schemas. Since there are three types of schema, there
are two types of mapping:
•• External/conceptual defines the correspondence between an external
schema and the conceptual schema.
•• Conceptual/internal defines the correspondence between the conceptual
and the internal level.
Figure 2.9 illustrates these ideas with a simple database maintaining
information about a company's employees. Two external views of the
database are considered in this example; one for the Finance Department and
one for the switchboard. The information needed by the Finance Department
uses the full name, age and salary of each employee and is defined by its
corresponding external schema. Each employee must be uniquely identified.
Since two employees might have the same name, a unique identifier, ID, is
The information needed by the switchboard, defined by the respective
schema, only refers to the name, job and telephone number of each
employee. The switchboard needs employees' first and last names to
be separate, so they can be sorted easily. For the switchboard, unique
identification of each employee is less critical, and the job title combined with
the employee's name is used. The assumption that two employees having
the same name will not have the same job title as well is considered safe
Other external views might exist too, and other data might be stored in the
database but we are dealing with a heavily simplified example for the sake of
The conceptual schema, that defines the conceptual level, unites the data
required to support the two views. It specifies the identifier, first and last
name, date of birth, job title, employment date, salary scale and the telephone
number of each employee.