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Database Management Systems/Managing Database

Notes Here, we can note that, one employee can work in many departments, or, one department can
have many employees. This can also be illustrated with the following Figure 6.4:

Figure 6.4

1/1/91
51
123 - 22 -3666 3/3/93
231 - 31 5368 2/2/92
56
131 - 24 -3650 3/1/92

223 - 32 -6316 62
3/1/92

Here, employee 231-31-5368 has worked in Department 51 since, 3/3/93 and in Department 56
since 2/2/92. Thus one department can have many employees.
But, if we want to have only one employee in a department, then it is an example of a Key
Constraint.

Example: Consider another relationship Manages between employees and department
entities as in the following figure.
Figure 6.5

ename Since dname

eno salary did budget

Employees Manages Department

Here, each department can have only one manager. The restriction that each department can
have only one manager is an example is an example of a Key Constraints This restriction is
indicated in the above ER diagram by using an arrow from department to manages, such that a
department can have only one manager. Therefore, as there is no arrow from employees entity
to manages relationship, we can say that, many employees or one employee can manage
department. But, as there is an arrow from department entity to manages relationship, we say
that, the departments is managed by only one employee or there is only one manager for
department(s), indicating many-to-one relationship.

Participation Constraint

The key constraint and its figure in the previous unit on manages relationship tells us that every
department has one manager. This requirement is an example of a participation constraints i.e.,

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