MySQL - Trigger - Before Insert and using the SK (Auto Increment) - mysql

Question

I have a simple posts table in MySQL which has a POST_ID as the SK (surrogate key).
Replies to the original post ID are stored in the same table in a PARENT_POST_ID column, but I want to perform the following logic:
BEFORE INSERT (I think ...)
IF a PARENT_POST_ID has not been defined on the INSERT, then default the row value to the newly generated POST_ID (from the auto-int sequence)
IF a PARENT_POST_ID has been defined on the INSERT, then set it to whatever has been passed.
Example
post_id | parent_post_id | date_time | message
12 12 2015-04-14 21:10 A new post (start of a thread)
13 12 2015-04-14 21:12 A reply to the post ID 12
The answer here: https://stackoverflow.com/a/11061766/1266457 looks like it might be what I need to do, although I am not sure what it's doing.
Thanks.

Answer 1

For before insert trigger you can not get the last inserted primary key , the other way of doing it is to get the max value from the table and increment it.
Here is a way to do it
delimiter //
create trigger posts_before_ins before insert on posts
for each row
begin
declare last_id int;
if new.parent_post_id is null then
select max(post_id) into last_id from posts ;
if last_id is null then
set new.parent_post_id = 1 ;
else
set new.parent_post_id = last_id+1 ;
end if ;
end if ;
end ;//
delimiter ;
So the trigger will check if there is no value of parent_post_id in the insert query it will get the max post_id. For the first entry it will be null so we are setting it as 1 i.e. and after that max post_id + 1 after each entry.
Here is a test case of this in mysql
mysql> select * from test ;
Empty set (0.00 sec)
mysql> delimiter //
mysql> create trigger test_is before insert on test
-> for each row
-> begin
-> declare last_id int;
-> if new.parent_id is null then
-> SELECT auto_increment into last_id
-> FROM INFORMATION_SCHEMA.TABLES WHERE table_name = 'test'
-> and TABLE_SCHEMA = 'test';
-> set new.parent_id = last_id ;
-> end if ;
-> end ;//
Query OK, 0 rows affected (0.12 sec)
mysql>
mysql> delimiter ;
mysql> insert into test (val) values ('aa');
Query OK, 1 row affected (0.10 sec)
mysql> insert into test (val) values ('bb');
Query OK, 1 row affected (0.04 sec)
mysql> select * from test ;
+---------+-----------+------+
| post_id | parent_id | val |
+---------+-----------+------+
| 1 | 1 | aa |
| 2 | 2 | bb |
+---------+-----------+------+
2 rows in set (0.00 sec)