SQL before insert trigger

I am trying to create a trigger but I'm running into trouble on a certain aspect. What I want the trigger to do is check the stock levels of a product being inserted, if the stock levels = 0 then the trigger will prevent the insert, if the level is between two values it will allow the insert but display a warning and lastly if the level is above a value it will insert no questions asked.

My problem is extracting the information from the insert in order to obtain the correct value for the trigger variable @stock_level.

Below I have a sample insert statement that will be retrieved from a GUI and below is the trigger I plan on using. I have entered in the required information into the trigger by hand but is it possible to extract this information without hard coding in the values?

Insert Statement

  --sample insert
  insert into sales_table values (1,2,3,4, '12/31/2015', '12:30:21');

Trigger

  create or replace trigger inv_check
  before insert on SALES_TABLE
  --declaring variable that contains the stock number of the product being inserted
  --4 is the product code and 1 is the outlet_number taken from the above
  DECLARE @stock_level = (select quantity from inventory_table where product_code = 4 and outlet_number = 1);
  BEGIN

  IF @stock_level = 0 then
    DBMS_OUTPUT.PUT_LINE("Sorry, out of stock");
    ROLLBACK;
  ELSIF @stock_level > 0 and @stcok_level < 15 then
    DBMS_OUTPUT.PUT_LINE("Running low order more");
    --continue with insert
  ELSE
    DBMS_OUTPUT.PUT_LINE("Inserting row...");
  END;

Solution

You don't need to declare the variable at all. In this case, the value you tried to assign to the variable is stored in :new.stock_level, such is the case with triggers. Here is a fix for the code you posted

CREATE OR REPLACE TRIGGER inv_check
BEFORE INSERT ON sales_table
BEGIN
    IF :new.stock_level == 0 THEN 
        DBMS_OUTPUT.PUT_LINE("Sorry, out of stock");
        ROLLBACK;
    ELSE IF :new.stock_level > 0 AND :new.stock_level < 15 THEN
        DBMS_OUTPUT.PUT_LINE("Running low order more");
        --continue with insert
    ELSE
        DBMS_OUTPUT.PUT_LINE("Inserting row...");
    END IF;
END;