MySQL触发器从一个表中添加/减少，取决于另一个表中的条目

Question

So, I have two tables: Transfers and Inventory.

What I am trying to do is when something is entered into the Transfer table, it will change an entry in the Inventory table.

An example of what I need is:

Say there is a man named Dr Jones and another man named Dr Smith. Dr Jones has £30 in his 'Total' and Dr Smith has £10. Now, say Dr Jones gives Dr Smith £10, I need the Inventory table to -£10 on Dr Jones' account total, and +£10 onto Dr Smith's account total. So therefore when you refresh the table, it states Dr Smith's balance in the total column as £20, and Dr Jones' balance as £20 now as well.

I want to do this through a trigger. I currently have the following:

DELIMITER $$
CREATE TRIGGER update_inventory_through_transfer AFTER INSERT ON Transfers
FOR EACH ROW
BEGIN
UPDATE Inventory
SET Inventory.total = SELECT SUM (IF (owner = NEW.owner, -total, total)) FROM Transfers WHERE owner = NEW.owner
END$$
DELIMITER;

Could anyone help me with where i'm going wrong? Thanks

Answer 1

The structure of your db is not clear from your question but assuming transfer knows the owner and who funds are being transferred to then the trigger should contain 2 straightforward updates, since a trigger is FOR EACH ROW then there is no need so sum the transfers.

drop table if exists transfer,inventory;

create table transfer (owner int,total int,tfr_to int);
create table inventory (owner int,total int);

insert into inventory values
(1,30),(2,10);

drop trigger if exists t; 
DELIMITER $$
CREATE TRIGGER t AFTER INSERT ON transfer
FOR EACH ROW
BEGIN
    UPDATE inventory t1
        SET t1.total = t1.total + new.total 
    where t1.owner = new.tfr_to;
    update inventory t1
        set t1.total = t1.total - new.total
    where t1.owner = new.owner;
END $$
DELIMITER ;

insert into transfer values (1,10,2);

select * from transfer;

+-------+-------+--------+
| owner | total | tfr_to |
+-------+-------+--------+
|     1 |    10 |      2 |
+-------+-------+--------+
1 row in set (0.02 sec)

select * from inventory;

+-------+-------+
| owner | total |
+-------+-------+
|     1 |    20 |
|     2 |    20 |
+-------+-------+
2 rows in set (0.00 sec)