Question

I managed to solve it using UNION between two querys, I belive my attempt was a little off and tried to do a mathematical add. This is problaby not the best way you can do it, but it worked and that’s enough for me. Thank you for your help.

Working solution:

CREATE VIEW Registrations AS
(SELECT S.identificationnumber AS StudentId, S.name AS StudentName, C.code AS CourseCode, C.name AS CourseName, 'Waiting' AS Status
FROM  Waitinglist W, Student S, Course C
WHERE S.identificationnumber = W.identificationnumber
AND W.code = C.code) UNION (SELECT S.identificationnumber AS StudentId, S.name AS StudentName, C.code AS CourseCode, C.name AS CourseName, 'Registered' AS Status
FROM  Registeredat R, Student S, Course C
WHERE S.identificationnumber = R.identificationnumber
AND R.code = C.code);

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Origianl Problem:

I’m a begginner at databases and SQL, so things might not look that professional.

What I’m trying to do in plain text:
I’m trying to create a view for all registerd and waiting students, for all courses. I also want to add a new “column” thats either “registerd” or “waiting”.

How I want the view to look:

StudentID, StudentName, CourseCode, CourseName, Status

StudentID = Combined idenficationnumber for Table "RegisterdAt" and "Waitinglist"
StudentName = Based on StudentID find matching name in Table "Student"
CourseCode = Combined code for Table "RegisterdAt" and "Waitinglist"
CourseName = based on code find matching name in Table "Course"
Status = Either "registered" or "waiting" 
   depending on if we got the "row" from Table "RegisterdAt" or "Waitinglist" 

The Created Tables(I have also added some examplery data into them, for easier testing):

CREATE TABLE Student(
identificationnumber  VARCHAR(20),
name VARCHAR(50),
branchname VARCHAR(50),
programmename VARCHAR(50),
PRIMARY KEY(identificationnumber),
FOREIGN KEY(branchname, programmename) REFERENCES Branch(name, programmename)
);

CREATE TABLE Course(
code CHAR(6),
name VARCHAR(50),
credits VARCHAR(10),
departmentname VARCHAR(50),
PRIMARY KEY(code),
FOREIGN KEY(departmentname) REFERENCES Department(name)
);

CREATE TABLE Waitinglist(
identificationnumber VARCHAR(20),
code CHAR(6),
ddate VARCHAR(10),
PRIMARY KEY(identificationnumber, code),
FOREIGN KEY(identificationnumber) REFERENCES Student(identificationnumber),
FOREIGN KEY(code) REFERENCES Course_with_maxstudents(code)
);

CREATE TABLE Registeredat(
identificationnumber VARCHAR(20),
code CHAR(6),
PRIMARY KEY(identificationnumber,code),
FOREIGN KEY(identificationnumber) REFERENCES Student(identificationnumber),
FOREIGN KEY(code) REFERENCES Course(code)
);

An attempt to create a view(not working, and missing registerd/waiting attribute):

CREATE VIEW Registrations AS
SELECT (R.identificationnumber + W.identificationnumber) AS StudentId, S.name AS StudentName, (R.code + W.code) AS CourseCode, C.name as CourseName
FROM  Registeredat R, Waitinglist W, Student S, Course C
WHERE S.identificationnumber = (R.identificationnumber + W.identificationnumber)
AND C.code = (R.code + W.code);

Answer 1

The working solution you posted looks great. I would just make the plain UNION into a UNION ALL, for it seems unlikely for you to need to remove duplicates from between these 2 subqueries. The ALL will prevent the server from doing unnecessary work to resort the combined results and search for non-existant duplicates.

So it would become:

CREATE VIEW Registrations AS  
(
    SELECT S.identificationnumber AS StudentId, S.name AS StudentName, C.code AS CourseCode, C.name AS CourseName, 'Waiting' AS Status  
    FROM  Waitinglist W, Student S, Course C  
    WHERE S.identificationnumber = W.identificationnumber  
    AND W.code = C.code  
)  
UNION ALL  
(  
    SELECT S.identificationnumber AS StudentId, S.name AS StudentName, C.code AS CourseCode, C.name AS CourseName, 'Registered' AS Status  
    FROM  Registeredat R, Student S, Course C  
    WHERE S.identificationnumber = R.identificationnumber  
    AND R.code = C.code  
);