How to use triggers in Mysql

Question

Guys I'm exploring triggers in MySQL and I'm a bit confused. I have created this tables:

persons

pid #primary key
last_name
first_name

then this table:

employee_hierarchy

emp_id #primary key
pid #foreign key
role

And created this trigger:

CREATE TRIGGER after_person_insert 
AFTER INSERT ON persons 
FOR EACH ROW 
INSERT INTO employee_hierarchy 
VALUES('','LAST_INSERT_ID()','employee');

TRIGGER Successfully created. But whenever I try to insert to persons table, i got a foreign key something error. So I dropped the foreign key and it was a success. But whats the use of this trigger thingy? Can you site a good example where this is useful? cause maybe this is not designed to update foreign keys, i dont know.

Answer 1

Instead of trying to get a value from LAST_INSERT_ID() (BTW you insert a string since it is in quotes) you just need need to read assigned value from NEW.pid . It is available to you at this point AFTER INSERT of time.

CREATE TRIGGER after_person_insert 
AFTER INSERT ON persons 
FOR EACH ROW 
  INSERT INTO employee_hierarchy 
  VALUES(NULL, NEW.pid, 'employee');

Here is SQLFiddle demo