如何使用GROUP BY在sql中触发？ （Oracle数据库）

Question

I want to make a trigger which sum the TOTALCOST based on the CUSTOMER_ID. And insert the data to the MAIN_ORDER table.

TABLE: ORDER_DETAIL

        ID |  CUSTOMER_ID | TOTALCOST 
          ------------------------  
          1 |    1000   | 50
          2 |    1000   | 50
          3 |    2000   | 200
          4 |    2000   | 200

TABLE: MAIN_ORDER

         CUSTOMER_ID | ORDER_PRICE 
          ------------------------  
           1000   | 100
           2000   | 400

I have tried this but it doesn't work

CREATE OR REPLACE TRIGGER MOVEDETAIL
BEFORE INSERT ON MAIN_ORDER FOR EACH ROW
BEGIN
SELECT ORDER_DETAIL.CUSTOMER_ID,ORDER_DETAIL.SUM(TOTALCOST)
INTO :NEW.CUSTOMER_ID,:NEW.ORDER_PRICE
FROM ORDER_DETAIL
GROUP BY CUSTOMER_ID;
END;

Answer 1

Keeping aside your trigger, if I understand well you have a table ORDER_DETAIL and you want to keep the data of a second table ( MAIN_ORDER ) aligned with every modification on ORDER_DETAIL data, so that MAIN_ORDER always contains the sum of TOTALCOST for each CUSTOMER_ID in ORDER_DETAIL .

If you want to do so with triggers, you have to handle every modification on ORDER_DETAIL , that is you need triggers for INSERT , UPDATE , DELETE on ORDER_DETAIL ( not on MAIN_ORDER ).

Given that you only have to keep informations aligned, I would suggest creating a view instead of handling a table with triggers.

For example:

CREATE OR REPLACE VIEW V_MAIN_ORDER
AS SELECT CUSTOMER_ID,
          SUM(TOTALCOST) AS ORDER_PRICE
   FROM ORDER_DETAIL
   GROUP BY CUSTOMER_ID

The view always shows correct informations, with no need for triggers

SQL> INSERT INTO ORDER_DETAIL( ID, CUSTOMER_ID, TOTALCOST) VALUES (1, 1000, 50);

1 row created.

SQL> INSERT INTO ORDER_DETAIL( ID, CUSTOMER_ID, TOTALCOST) VALUES (2, 1000, 50);

1 row created.

SQL> INSERT INTO ORDER_DETAIL( ID, CUSTOMER_ID, TOTALCOST) VALUES (3, 2000, 200);

1 row created.

SQL> INSERT INTO ORDER_DETAIL( ID, CUSTOMER_ID, TOTALCOST) VALUES (4, 2000, 200);

1 row created.

SQL> SELECT * FROM V_MAIN_ORDER;

CUSTOMER_ID ORDER_PRICE
----------- -----------
       1000         100
       2000         400

and no matter what you do on the table

SQL> DELETE ORDER_DETAIL WHERE CUSTOMER_ID = 1000;

2 rows deleted.

SQL> SELECT * FROM V_MAIN_ORDER;

CUSTOMER_ID ORDER_PRICE
----------- -----------
       2000         400

SQL>