Mysql trigger multiply copies to another table - mysql

Question

I have a trigger which copies the inserted data from my table 1 to table 2. I tried inserting 2 data into table 1 and it copies it to my table 2 but the the problem is it copies 2 times the data. Whenever I insert A, B, C into table 1 the value being copied in my table 2 is A, A, A, B, B, B, C, C, C. Please help.
delimiter //
CREATE TRIGGER `copy_table` AFTER INSERT ON table_2
insert into table_1 (id, code, name)
select id, code, name
from table_2;
END;
//
delimiter ;

Answer 1

Your problem is that you are inserting all the data from table_2 into table_1 each time your trigger executes. You need to only insert the new values, which you can do by referring to the NEW pseudo-table:
delimiter //
CREATE TRIGGER `copy_table` AFTER INSERT ON table_2
FOR EACH ROW
BEGIN
INSERT INTO table_1 (id, code, name)
VALUES (NEW.id, NEW.code, NEW.name);
END;
//
delimiter ;
Demo on dbfiddle
Update
If you want the trigger to only insert values if they don't already exist in table_1, you can either
add a UNIQUE index on (id, code, name) to table_1 and use INSERT IGNORE in the trigger; or
Update the trigger to check that the data doesn't exist in the table already:
CREATE TRIGGER `copy_table` AFTER INSERT ON table_2
FOR EACH ROW
BEGIN
IF NOT EXISTS (SELECT *
FROM table_1
WHERE id = NEW.id AND code = NEW.code AND name = NEW.name) THEN
INSERT INTO table_1 (id, code, name) VALUES (NEW.id, NEW.code, NEW.name);
END IF;
END;
Demo on dbfiddle