SQL：觸發語法

Question

我正在做一些課程工作，我應該創建一個觸發器，在創建訂單之前檢查庫存水平，以查看庫存中是否有足夠的產品來滿足訂單。問題是講師沒有正確地教導我們創建觸發器的語法，我的問題是我不知道如何告訴觸發器我正在檢查的stocktable中的哪個行/列。我需要加入表格嗎？

這是我到目前爲止的代碼（我只是即興表演在這裏，它並不真正的工作，但我把它在這裏給你的是什麼，我想acchieve一個想法）：

CREATE OR REPLACE TRIGGER stock_check 
BEFORE INSERT OR UPDATE OF product_quantity ON orderline 
FOR EACH ROW 
DECLARE 
Newtotal number; 
insufficient_stock exception; 
BEGIN 
IF(:NEW.product_quantity > product_stock.stock_quantity) then 
raise insufficient_stock; 
Else 
:new.product_stock.stock_quantity - product_quantity = newtotal 
Update stock_quantity 
Set product_quantity = newtotal 


END IF; 
exception 
when insufficient_stock then 
raise_application_error(-20604,'There is not enoguh stock available'); 
END; 

Answer 1

我想你需要的東西像下面這樣：

CREATE OR REPLACE TRIGGER stock_check 
    BEFORE INSERT ON orderline 
    FOR EACH ROW 
DECLARE 
    oldtotal NUMBER 
    newtotal NUMBER; 
    insufficient_stock EXCEPTION; 
BEGIN 
    SELECT stock_quantity INTO old_total 
    FROM product_stock 
    WHERE product_id = :new.product_id: 

    IF(:new.product_quantity > oldtotal) then 
    RAISE insufficient_stock; 
    ELSE 
    newtotal := oldtotal - :new.product_quantity;   
    UPDATE stock_quantity 
     SET product_quantity = newtotal 
    WHERE product_id = :new.product_id; 
    END IF; 
EXCEPTION 
    WHEN insufficient_stock THEN 
    RAISE_APPLICATION_ERROR(-20604,'There is not enoguh stock available'); 
END; 

您仍然需要添加一列用於某種PRODUCT_ID的。我沒處理UPDATE情況下...

Answer 2

例如：

SQL> create table product_stock (prod_id number, stock_quantity number); 

Table created. 

SQL> create table orderline (prod_id number, product_quantity number); 

Table created. 

SQL> insert into product_stock values (1, 1002); 

1 row created. 

SQL> 
SQL> CREATE OR REPLACE TRIGGER stock_check 
    2 BEFORE INSERT OR UPDATE OF product_quantity ON orderline 
    3 FOR EACH ROW 
    4 DECLARE 
    5 v_stock_quantity product_stock.prod_id%type; 
    6 v_Newtotal number; 
    7 insufficient_stock exception; 
    8 BEGIN 
    9 if updating 
10  then 
11  v_Newtotal := :new.product_quantity - :old.product_quantity; 
12  else 
13    v_Newtotal := :new.product_quantity; 
14  end if; 
15 
16  Update product_stock 
17  Set stock_quantity = stock_quantity - v_Newtotal 
18    where prod_id = :new.prod_id 
19    returning stock_quantity into v_stock_quantity; 
20 
21 IF (v_stock_quantity < 0) 
22 then 
23  raise insufficient_stock; 
24 END IF; 
25 exception 
26 when insufficient_stock then 
27  raise_application_error(-20604,'There is not enoguh stock available'); 
28 END; 
29/

Trigger created. 

SQL> show errors 
No errors. 
SQL> select * from product_stock; 

    PROD_ID STOCK_QUANTITY 
---------- -------------- 
     1   1002 

SQL> insert into orderline values (1, 1000); 

1 row created. 

SQL> select * from product_stock; 

    PROD_ID STOCK_QUANTITY 
---------- -------------- 
     1    2 

SQL> insert into orderline values (1, 3); 
insert into orderline values (1, 3) 
      * 
ERROR at line 1: 
ORA-20604: There is not enough stock available 
ORA-06512: at "DTD_TRADE.STOCK_CHECK", line 17 
ORA-04088: error during execution of trigger 'DTD_TRADE.STOCK_CHECK' 


SQL> select * from product_stock; 

    PROD_ID STOCK_QUANTITY 
---------- -------------- 
     1    2 

更新完成第一個故意創建一個鎖（好像兩會在曾經做過這個電話，你想要的第二爲了避免庫存低於0）。