I need a help to write a function in an oracle.

if i have following table how can I write a function Employees.Salary must fit between min_salary and max_salary in the jobs table, if min_salary and max_salary are not null.

and Jobs.Min_Salary must be <= max_Salary, if not null.

CREATE TABLE DEPARTMENT (DEPARTMENT_ID NUMBER PRIMARY KEY, DEPARTMENT_NAME VARCHAR(30) NOT NULL );

CREATE TABLE JOBS (JOB_ID NUMBER PRIMARY KEY, JOB_TITLE VARCHAR(35) NOT NULL, MIN_SALARY DECIMAL NOT NULL, MAX_SALARY DECIMAL NOT NULL );

CREATE TABLE EMPLOYEES (EMPLOYEE_ID NUMBER PRIMARY KEY, FIRST_NAME VARCHAR(20) NOT NULL, LAST_NAME VARCHAR(25) NOT NULL, EMAIL VARCHAR(25) NOT NULL, PHONE_NUMBER VARCHAR(20) NOT NULL, HIRE_DATE DATE NOT NULL, JOB_ID NUMBER NOT NULL, SALARY DECIMAL NOT NULL, DEPARTMENT_ID NUMBER NOT NULL, CONSTRAINT emp_job_fk FOREIGN KEY(JOB_ID) REFERENCES JOBS(JOB_ID), CONSTRAINT emp_department_fk FOREIGN KEY(DEPARTMENT_ID) REFERENCES DEPARTMENT(DEPARTMENT_ID) );

INSERT INTO DEPARTMENT (DEPARTMENT_ID,DEPARTMENT_NAME) VALUES(1,'IT'); INSERT INTO DEPARTMENT (DEPARTMENT_ID,DEPARTMENT_NAME) VALUES(2,'Sales');

INSERT INTO JOBS (JOB_ID,JOB_TITLE,MIN_SALARY,MAX_SALARY) VALUES (1,'IT Administrator',250000.00,50000.00); INSERT INTO JOBS (JOB_ID,JOB_TITLE,MIN_SALARY,MAX_SALARY) VALUES (2,'Salesman',200000.00,40000.00);