How do we implement an IS-A Relationship?

I did as Donnie suggested, but without the role field, because it complicates things. This is the final implementation:

DDL:

CREATE TABLE Employee (
ast VARCHAR(20) not null,
firstname VARCHAR(200) not null,
surname VARCHAR(200) not null,
...
PRIMARY KEY(ast)
);

CREATE TABLE Administrative (
employee_ast VARCHAR(20) not null REFERENCES Employee(ast),
PRIMARY KEY(employee_ast)
);

CREATE TABLE Technical (
employee_ast VARCHAR(20) not null REFERENCES Employee(ast),
...
PRIMARY KEY(employee_ast)
);

ER Diagram:

In this model there are no Employees of Generic Type. Here, an Employee can only be Administrative or Technical.