创建 Oracle 触发器以根据属性在另一个表中的唯一性来增加/减少属性

Question

我正在制作一个数据库来为游戏行业建模，它有两个看起来有点像这样的表：

PLAYABLE (Which games are playable on which consoles)
--------------------------------------------------
|  Game_ID (int)   | Console_Name (varchar(255)) | (both primary/foreign)
--------------------------------------------------

 CONSOLE
--------------------------------------------------
|  Console_Name (PK) |     Exclusives (int)      |
--------------------------------------------------

exclusives 属性给出了该控制台（并且只有该控制台）拥有的游戏数量。

因此，当我在“Playable”关系中插入一个新元组时，如果该“Game_ID”已经存在一次，那么我需要使用相应的“Console_Name”递减“Exclusives”属性。同样，如果它不存在，那么我会增加该控制台的独占数量。

我是 Oracle 新手，这是我为此创建触发器的尝试：

CREATE OR REPLACE TRIGGER Exclusives_Count
    BEFORE INSERT ON Playable
    FOR EACH ROW
BEGIN
    DECLARE
        Game_Count NUMBER;
        Obsolete_Exclusive_Console VARCHAR(255);
    SELECT COUNT(Game_ID) INTO Game_Count FROM Playable WHERE Game_ID = :NEW.Game_ID;
    IF Game_Count = 0
    THEN 
        UPDATE Console
        SET Exclusives = Exclusives + 1
        WHERE Console.Console_Name = :NEW.Console_Name;
    ELSIF Game_Count = 1
    THEN
        SELECT Console_Name INTO Obsolete_Exclusive_Console FROM Playable WHERE Game_ID = :NEW.Game_ID;

        UPDATE Console
        SET Exclusives = Exclusives - 1
        WHERE Console.Console_Name = Obsolete_Exclusive_Console;
    END IF;
END;
/

输入这个会给我“警告：触发器创建时出现编译错误”，但我不确定我哪里出错了。

Answer 1

declare 应该在您的代码中的 begin 之前 请参阅下面的声明：

create table Playable(Game_ID int, Console_Name varchar(255));
create table CONSOLE(Console_Name  varchar(255), Exclusives int);
SQL> 
SQL> CREATE OR REPLACE TRIGGER Exclusives_Count
  2      BEFORE INSERT ON Playable
  3      FOR EACH ROW
  4  
  5  DECLARE
  6          Game_Count NUMBER;
  7          Obsolete_Exclusive_Console VARCHAR(255);
  8  BEGIN
  9  
 10      SELECT COUNT(Game_ID) INTO Game_Count FROM Playable WHERE Game_ID = :NEW.Game_ID;
 11      IF Game_Count = 0
 12      THEN
 13          UPDATE Console
 14          SET Exclusives = Exclusives + 1
 15          WHERE Console.Console_Name = :NEW.Console_Name;
 16      ELSIF Game_Count = 1
 17      THEN
 18          SELECT Console_Name INTO Obsolete_Exclusive_Console FROM Playable WHERE Game_ID = :NEW.Game_ID;
 19  
 20          UPDATE Console
 21          SET Exclusives = Exclusives - 1
 22          WHERE Console.Console_Name = Obsolete_Exclusive_Console;
 23      END IF;
 24  END;
 25  /

Trigger created

SQL> show errors
No errors for TRIGGER SCM.EXCLUSIVES_COUNT

SQL>