Oracle Inserting a row in a table only if a match found in some other table - oracle

Question

I have 3 tables in my database namely employees,students and Images
create table employees(id number primary key,name varchar2(100), address varchar2(100));
create table students(id number primary key,name varchar2(100),address varchar2(100));
create table Images (image_id number primary key,employee_id number,student_id number,image_name varchar2(100));
Insert into employees values (1,'asdfasd','asdfasdf');
Insert into employees values (2,'asdfasd','asdfasdf');
Insert into employees values (3,'asdfasd','asdfasdf');
Insert into employees values (4,'asdfasd','asdfasdf');
Insert into employees values (5,'asdfasd','asdfasdf');
Insert into students values (1,'asdfasd','asdfasdf');
Insert into students values (2,'asdfasd','asdfasdf');
Insert into students values (3,'asdfasd','asdfasdf');
Insert into students values (4,'asdfasd','asdfasdf');
Insert into students values (5,'asdfasd','asdfasdf');
Insert into students values (49,'asdfasd','asdfasdf');
Insert into Images(image_id,employee_id,image_name) values (1,5,'adsfasdfasdf');
Insert into Images(image_id,student_id,image_name) values (2,49,'asfasdfasdf');
Now, when Inserting a row into the Images table I should check whether the employee_id / Student_id is existed in the employees table/student table, If a match found then only I have to Insert else it should not.
I thought of adding two foreign keys on the images table as follows:
alter table images add constraint fk_employeeid foreign key(employee_id)
references employees(id);
alter table images add constraint fk_studentsid foreign key(student_id)
references students(id);
But, If I do so. It will not allow me to insert null values. How can I modify my design so that whenever I insert a row in images table either it should be an employee_id or an student_id.
If I created any confusion, here is the link for the sql fiddle,http://www.sqlfiddle.com/#!4/92d24/1/0

Answer 1

I thought of adding two foreign keys on the images table.
But, If I do so. It will not allow me to insert null values.
You are wrong when you say the foreign key constraint won't allow NULL values. It will definitely allow the NULL values.
Test case
Let's add the foreign key constraint on the IMAGES table for the employee_id and student_id.
ALTER TABLE images ADD CONSTRAINT fk_emp FOREIGN KEY(employee_id) REFERENCES employees(ID);
ALTER TABLE images ADD CONSTRAINT fk_stu FOREIGN KEY(student_id) REFERENCES students(ID);
Let's check the INSERT with NULL values:
SQL> INSERT INTO Images(image_id,employee_id,image_name) VALUES (1,5,'adsfasdfasdf');
1 row created.
SQL> INSERT INTO Images(image_id,student_id,image_name) VALUES (2,49,'asfasdfasdf');
1 row created.
SQL> INSERT INTO Images(image_id,employee_id,image_name) VALUES (3,null,'adsfasdfasdf');
1 row created.
SQL> INSERT INTO Images(image_id,student_id,image_name) VALUES (4,null,'asfasdfasdf');
1 row created.
SQL>
SQL> COMMIT;
Commit complete.
SQL>
SQL> SELECT * FROM images;
IMAGE_ID EMPLOYEE_ID STUDENT_ID IMAGE_NAME
---------- ----------- ---------- ---------------
1 5 adsfasdfasdf
2 49 asfasdfasdf
3 adsfasdfasdf
4 asfasdfasdf
SQL>
Let's check the foreign key constraint validation:
SQL> INSERT INTO Images(image_id,employee_id,image_name) VALUES (1,10,'adsfasdfasdf');
INSERT INTO Images(image_id,employee_id,image_name) VALUES (1,10,'adsfasdfasdf')
*
ERROR at line 1:
ORA-00001: unique constraint (LALIT.SYS_C0010739) violated
SQL> INSERT INTO Images(image_id,student_id,image_name) VALUES (2,20,'asfasdfasdf');
INSERT INTO Images(image_id,student_id,image_name) VALUES (2,20,'asfasdfasdf')
*
ERROR at line 1:
ORA-00001: unique constraint (LALIT.SYS_C0010739) violated
SQL>
So, everything works as per the design.

Answer 2

Add constraints to images like you did. Add one more constraint:
alter table images add constraint chk_nulls
check (
(employee_id is not null and student_id is null)
or (employee_id is null and student_id is not null)
);
This way you cannot insert both nulls nor both not nulls for employee_id and student_id. And foreign keys are also checked.
Test:
insert into images values (1, 1, null, 'not important'); -- OK
insert into images values (2, null, null, 'not important'); -- error
insert into images values (3, 1, 1, 'not important'); -- error
insert into images values (4, null, 1, 'not important'); -- OK

Answer 3

I would keep the foreign keys, that is the correct approach.
But instead of inserting NULL's, you should define an "unknown record" in your EMPLOYEES and STUDENTS tables (maybe with an id=-1, name='UNKNOWN'), and insert -1 into your IMAGES table.

Answer 4

I recommend you to change the primary key in table images, and have as PK three columns (image_id, employee_id and student_id).
Edit 1: Based on comment
I think you planned in a wrong way the problem. If the images are for both, employees and students, you should have two tables, images_employees and images_students, with respectively foreign keys employee_id and student_id and of course an id for the image.
But is there any sense to have a row like this?
id: 1 student_id: null image_name: 'something'
I don't think so... please explain more about the purpose of images table.