創建視圖+查詢（組合列+添加額外的屬性）

Question

我設法解決它在兩個查詢之間使用UNION，我相信我的嘗試是有點關閉，並試圖做一個數學增加。這是problaby不是你可以做到的最好方式，但它有效，對我來說就足夠了。感謝您的幫助。

工作液：

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); 

---

原著的問題：

我在數據庫和SQL一個begginner，這樣的事情可能不看那個專業。

我想用純文本做什麼： 我試圖爲所有註冊和等待學生創建所有課程的視圖。我還想添加一個新的「列」，即「註冊」或「等待」。

我怎麼想的觀點看：

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" 

的創建的表（我還添加了一些examplery數據放進去，更容易測試）：

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) 
); 

試圖創建視圖（不工作，缺少註冊/等待屬性）：

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

您發佈的工作解決方案看起來不錯。我只是把普通的UNION變成UNION ALL，因爲你似乎不太可能需要刪除這兩個子查詢之間的重複。 ALL將阻止服務器進行不必要的工作，以便合併結果並搜索不存在的重複項。

因此，這將成爲：

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 
); 

Answer 2

添加列狀態「註冊」，這取決於r.code是不爲空，否則「等待」

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, 
     case when r.code is not null then 'registered' else 'waiting' end as status 
FROM Registeredat R, Waitinglist W, Student S, Course C 
WHERE S.identificationnumber = (R.identificationnumber + W.identificationnumber) 
AND C.code = (R.code + W.code); 

SQL Fiddle進行進一步的測試。

我放棄了外鍵約束，因爲這裏沒有定義各種表。