创建视图+查询（组合列+添加额外的属性）

Question

我设法解决它在两个查询之间使用UNION，我相信我的尝试是有点关闭，并试图做一个数学增加。这是problaby不是你可以做到的最好方式，但它有效，对我来说就足够了。感谢您的帮助。

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); 

---

原著的问题：

我在数据库和SQL一个begginner，这样的事情可能不看那个专业。

我想用纯文本做什么： 我试图为所有注册和等待学生创建所有课程的视图。我还想添加一个新的“列”，即“注册”或“等待”。

我怎么想的观点看：

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" 

的创建的表（我还添加了一些examplery数据放进去，更容易测试）：

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) 
); 

试图创建视图（不工作，缺少注册/等待属性）：

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

您发布的工作解决方案看起来不错。我只是把普通的UNION变成UNION ALL，因为你似乎不太可能需要删除这两个子查询之间的重复。 ALL将阻止服务器进行不必要的工作，以便合并结果并搜索不存在的重复项。

因此，这将成为：

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

添加列状态“注册”，这取决于r.code是不为空，否则“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进行进一步的测试。

我放弃了外键约束，因为这里没有定义各种表。