can someone jelp me on this? - Mailing list pgsql-sql

hey guys I have a question that I couldnt maneged to solve for the last 4 
days. Im kinda new to these stuff so dont have fun with me if it was so 
easy. Ok now heres the question.


create table student
(student_id int not null,
student_name varchar(25),
student_affiliation int,
student_password varchar(20),
student_email varchar(30),
student_phone varchar(12),
student_address varchar(50));

create table instructor
(instructor_id int not null,
instructor_name varchar(25),
instructor_affiliation int,
instructor_password varchar(20),
instructor_email varchar(30),
instructor_phone varchar(12),
instructor_address varchar(50));

create table course
(course_id int not null,
course_name varchar(25),
course_sections int,
course_description varchar(50));

create table class
(course_id int not null,
class_section int not null,
class_strength int,
class_date date,
class_time time);

create table student_class
(course_id int not null,
class_section int,
student_id int not null);

create table instructor_class
(course_id int not null,
class_section int,
instructor_id int not null);

insert into student (student_id, student_name) Values (1, 'Marge Trechle');
insert into student (student_id, student_name) Values (2, 'Todd Relve');
insert into student (student_id, student_name) Values (3, 'Beth 
Schavinsky');
insert into student (student_id, student_name) Values (4, 'Thomas 
Newburry');
insert into student (student_id, student_name) Values (5, 'Geoff Hamilton');
insert into student (student_id, student_name) Values (6, 'Cindy Crumple');
insert into student (student_id, student_name) Values (7, 'Keith Snyder');
insert into student (student_id, student_name) Values (8, 'Belinda Myers');
insert into student (student_id, student_name) Values (9, 'Hope Wilson');
insert into student (student_id, student_name) Values (10, 'Icculus McMan');
insert into student (student_id, student_name) Values (11, 'Mary Calahan');

insert into instructor (instructor_id, instructor_name) values (1, 'Norm 
Salvadori');
insert into instructor (instructor_id, instructor_name) values (2, 'Terry 
Smith');

insert into course (course_id, course_name) values (1, 'Wetlands Critters');
insert into course (course_id, course_name) values (2, 'Ocean Ecosystems');
insert into course (course_id, course_name) values (3, 'Endangered 
Habitats');

insert into class (course_id, class_section) values (1,1);
insert into class (course_id, class_section) values (1,2);
insert into class (course_id, class_section) values (2,1);
insert into class (course_id, class_section) values (3,1);

The values inserted into the database can change and more entries can be 
inserted and in this situation I have to write the constraints according to 
these criteria:
1. A COURSE may consist of one or more CLASSes
2. An INSTRUCTOR may teach one or more CLASSes
3. A STUDENT may enroll in many CLASSes
4. A STUDENT can take more than one CLASS, and each CLASS contains many
STUDENTS
5. A CLASS is taught by only one INSTRUCTOR, but an INSTRUCTOR can teach
many CLASSes
6. A COURSE consists many CLASSes, while each CLASS is based on one
COURSE, so there is a one-to-many relationship between COURSE and
CLASS
7. A STUDENT may not enroll himself/herself for two CLASSes of the same
course
8. An INSTRUCTOR may not teach two CLASSes of the same COURSE

�f someone can help me about these stuff I will really apreciate.