Need to make date validation to remove overlab

Question

I have a problem in date validation
I will tell you the scenario
I have department table as a master table and under this department there is some teachers.
Eeach teacher have Start Hiring Date and End Hiring Date .
I want to prevent to insert a new teacher with Start Hiring date or End hiring date between any period inserted before in this department .
This means in any period there is only one hired teacher
how can I do this validation

Answer 1

Assumed that you are using ADF BC, I reproduced your use case, and tried this. you can also try it,
    CREATE TABLE test_department
    (      id      numeric(10)      not null,
         name      varchar2(50) not null,
         CONSTRAINT dpt_pk PRIMARY KEY (id)
    CREATE TABLE teachers
    (      id      numeric(10) PRIMARY KEY     ,
         dept_id      numeric(10)      not null,
          start_date date,
          end_date date,
         CONSTRAINT fk_emp  FOREIGN KEY (dept_id)
           REFERENCES test_department(id)
insert into test_department values (1,'Math');
insert into teachers values(1,1,to_date('1/1/2010','DD/MM/YYYY'),to_date('31/12/2010','DD/MM/YYYY'));add a validate entity method to you TeacherImpl class.
    public boolean validateTeachers()
        TestDepartmentImpl deptImpl = this.getTestDepartment(); // TestDepartment is the accessor name exposed in TeachersImpl class
        RowIterator iter = deptImpl.getTeachers(); //Teachers is the accessor name exposed in DepartmentImpl class
        boolean flag = true;
        while (iter.hasNext())
                TeachersImpl currentTeacher = (TeachersImpl)iter.next();
                if (!currentTeacher.getId().equals(this.getId())) //skip the current teacher from comparison
                    if (this.getStartDate().compareTo(currentTeacher.getStartDate()) >
                        0 && this.getEndDate().compareTo(currentTeacher.getEndDate())<0)
                          flag=false;
        return flag;
    }