PL/SQL編寫一個過程來更新列？

Question

我應該更新年級學生根據程序

ChangeGrade(p_sID, p_classID) 

如果學生沒有在類（p_classID）就讀「A」則打印錯誤消息。

下面是表：

招生報名

sID classID Grade 
*** ******* ***** 
104  10440  B 
102  10220  C 
...  .....  . 

我應該做一個內部聯接？這是我有什麼：

Create or Replace ChangeGrade (
    p_sID enrolling.sID%type 
    p_classID enrolling.classID%type) 
AS 
    p_id_enrolled NUMBER; 

BEGIN 
    SELECT sID into p_id_enrolled 
    FROM Enrolling 
    WHERE sID = p_sID 
    AND classID = p_classID 

    IF p_sID = p_id_enrolled THEN 
    update Enrolling 
    set GRADE = 'A' 
    dbms_output.put_line('Student grade has been changed.') 
    ELSE 
    dbms_output.put_line('Student record does not exist.') 
    END IF; 
END; 
/ 

Answer 1

這裏是你的代碼中的問題：

1. 這個查詢是沒有意義的：

   SELECT sID into p_id_enrolled 
       FROM Enrolling 
       WHERE sID = p_sID 
       AND classID = p_classID 
   

   您選擇sID爲p_id_enrolled，但在WHERE條款您篩選sID = p_sID，所以p_id_enrolled總是會等於p_sID，並且根本不需要此選擇語句。

2. 此更新語句將更新整個表：

   UPDATE Enrolling 
       SET GRADE = 'A'; 
   

   您需要添加一個過濾子句只更新一行。

3. 如果學生沒有在課程中註冊，則查詢不會返回任何行，您將得到NO_DATA_FOUND例外。要處理它，你需要捕捉異常或計算學生數量。

我會建議使用下列內容：

Create or Replace procedure ChangeGrade (
    p_sID enrolling.sID%type, 
    p_classID enrolling.classID%type) 
AS 
    cnt NUMBER; 

BEGIN 
    SELECT count(*) into cnt 
    FROM Enrolling 
    WHERE sID = p_sID 
    AND classID = p_classID; 

    IF cnt = 1 THEN 
    update Enrolling 
     set GRADE = 'A' 
    where sID = p_sID 
     AND classID = p_classID; 
    dbms_output.put_line('Student grade has been changed.'); 
    ELSE 
    dbms_output.put_line('Student record does not exist.'); 
    END IF; 
END; 
/

Answer 2

我不知道打印出哪種錯誤，但它似乎像你缺少';'在一些命令的結尾處（在if語句和select into之後）。

Answer 3

正如我在評論中提到的，在您的程序中有幾個地方semicolon（line termator）缺失。試試這個：

CREATE OR REPLACE Procedure ChangeGrade (
    p_sID enrolling.sID%type 
    p_classID enrolling.classID%type) 
AS 
    p_id_enrolled NUMBER; 

BEGIN 
    SELECT sID 
    INTO p_id_enrolled 
    FROM Enrolling 
    WHERE sID = p_sID AND classID = p_classID; 

    IF p_sID = p_id_enrolled 
    THEN 
     UPDATE Enrolling 
     SET GRADE = 'A' 
     WHERE sID = p_sID; 

     DBMS_OUTPUT.put_line ('Student grade has been changed.'); 
    ELSE 
     DBMS_OUTPUT.put_line ('Student record does not exist.'); 
    END IF; 

EXCEPTION 
WHEN NO_DATA_FOUND Then 
dbms_output.put_line('Student record does not exist for this class'); 

END; 
/

Answer 4

您需要處理的事實，學生是不是在類爲例外：

Create or Replace PROCEDURE ChangeGrade (
    p_sID enrolling.sID%type, 
    p_classID enrolling.classID%type, 
    p_grade enrolling.grade%type ) 
AS 
l_enrolled NUMBER; 

BEGIN 
    SELECT sID INTO l_enrolled 
    FROM Enrolling 
    WHERE sID = p_sID 
    AND classID = p_classID; 

    IF l_enrolled = p_sID THEN 
    update Enrolling set GRADE = p_grade WHERE sID = p_sID and classID = p_classID; 
    dbms_output.put_line('Student grade has been changed.'); 
    END IF; 

    EXCEPTION WHEN NO_DATA_FOUND 
    dbms_output.put_line('Student record does not exist for this class'); 
END; 
/ 

我還添加了等級的參數，因爲它的邏輯把它作爲好。

Answer 5

Create or Replace procedure ChangeGrade (
    p_sID enrolling.sID%type 
    p_classID enrolling.classID%type) 
AS 
BEGIN 
    update Enrolling 
    set GRADE = 'A' 
    where sID = p_sID 
    AND classID = p_classID; 
    IF SQL%ROWCOUNT > 0 THEN 
    dbms_output.put_line('Student grade has been changed.'); 
    ELSE 
    dbms_output.put_line('Student record does not exist.'); 
    END IF; 
END; 

Answer 6

你也可以使用一個光標，然後看如果學生在課程就讀。如果他/她是，則更新等級。我認爲光標更安全，然後是SELECT X INTO n，因爲當您嘗試插入NULL,SELECT NULL INTO n時確實會出現一個難看的錯誤，而且這些錯誤通常很難找到。

例如：

CREATE OR REPLACE ChangeGrade (p_sID enrolling.sID%TYPE, 
           p_classID enrolling.classID%TYPE) 
AS 
       CURSOR cEnrolling IS 
        SELECT  * 
        FROM  Enrolling 
        WHERE  SID = p_sID 
        AND  classID = p_classID; 

       rEnrolling cEnrolling%ROWTYPE; 
BEGIN 
     OPEN cEnrolling; 
     FETCH cEnrolling INTO rEnrolling; 
       IF cEnrolling%FOUND THEN 
         -- Student record found. 
         UPDATE Enrolling 
         SET Grade = 'A' 
         WHERE SID = rEnrolling.sId; 

         DBMS_OUTPUT.PUT_LINE('Student grade has been changed.'); 
       ELSE 
         -- Student record not found. 
         DBMS_OUTPUT.PUT_LINE('Student record does not exist.'); 
       END IF; 
     CLOSE cEnrolling; 
END; 
/