Create a trigger to insert the old data to a new table - mysql

Question

Here is the table I created.
USE my_guitar_shop;
DROP TABLE IF EXISTS Products_Audit;
CREATE TABLE Products_Audit (
audit_id INT PRIMARY KEY,
category_id INT REFERENCES categories(category_id),
product_code VARCHAR ( 10 ) NOT NULL UNIQUE ,
product_name VARCHAR ( 255 ) NOT NULL,
list_price INT NOT NULL,
discount_percent INT NOT NULL DEFAULT 0.00 ,
date_updated DATETIME NULL);
Create a trigger named products_after_update. This trigger should insert the old data about the product into the Products_Audit table after the row is updated. Then, test this trigger with an appropriate UPDATE statement.
Here is the trigger I created but the data is not showing up in the Products_Audit table it is showing all null.
USE my_guitar_shop;
DROP TRIGGER IF EXISTS products_after_update;
DELIMITER $$
CREATE TRIGGER products_after_update
BEFORE UPDATE ON products
FOR EACH ROW
BEGIN
INSERT INTO products_audit (audit_id, product_id, category_id, product_code,
product_name, list_price, discount_percent, date_updated)
SELECT audit_id, products.product_id, products.category_id, products.product_code,
products.product_name,products.list_price, products.discount_percent, date_updated
FROM products JOIN products_audit
ON products_audit.audit_id = (SELECT audit_id FROM inserted);
END $$
DELIMITER ;
EDIT with the INSERT INTO
USE my_guitar_shop;
DROP TRIGGER IF EXISTS products_after_update;
DELIMITER $$
CREATE TRIGGER products_after_update
BEFORE UPDATE ON products
FOR EACH ROW
BEGIN
INSERT INTO products_audit (audit_id, product_id, category_id,product_code,
product_name, list_price, discount_percent, date_updated)
VALUES (OLD.audit_id, OLD.product_id, OLD.category_id, OLD.product_code,
OLD.product_name, OLD.list_price, OLD.discount_percent, OLD.date_updated)
DELIMITER ;

Answer 1

You are overcomplicating the insert. As mysql documentation on triggers says:
In an UPDATE trigger, you can use OLD.col_name to refer to the columns
of a row before it is updated and NEW.col_name to refer to the columns
of the row after it is updated.
Therfore, use the OLD.column_name format in the insert. Also, I would set the audit_id field to auto increment and leave it out of the insert:
INSERT INTO products_audit (product_id, category_id, product_code,
product_name, list_price, discount_percent, date_updated)
VALUES (OLD.product_id, OLD.category_id, OLD.product_code,
OLD.product_name, OLD.list_price, OLD.discount_percent, OLD.date_updated)

Answer 2

Here's an example of how I do it:
CREATE OR REPLACE EDITIONABLE TRIGGER E_TABLE_TRG
before insert or update or delete on e_table
for each row
declare
l_seq number;
begin
-- Get a unique sequence value to use as the primary key
select s_seq.nextval
into l_seq
from dual;
if inserting then
:new.date_opened := sysdate;
:new.last_txn_date := null;
:new.status := 'A';
end if;
if inserting then
insert into e_table_history
(
t_seq,
user_id,
date_opened,
last_txn_date,
status,
insert_update_delete,
insert_update_delete_date
)
values
(
l_seq,
:new.user_id,
:new.date_opened,
:new.last_txn_date,
:new.status,
'I',
sysdate
);
elsif updating then
insert into e_table_history
(
t_seq,
date_opened,
last_txn_date,
status,
insert_update_delete,
insert_update_delete_date
)
values
(
l_seq,
:new.date_opened,
:new.last_txn_date,
:new.status,
'U',
sysdate
);
else
insert into e_table_history
(
t_seq,
date_opened,
last_txn_date,
status,
insert_update_delete,
insert_update_delete_date
)
values
(
l_seq,
:old.date_opened,
:old.last_txn_date,
:old.status,
'D',
sysdate
);
end if;
end;
/
ALTER TRIGGER E_TABLE_TRG ENABLE;
/