Trigger update the same table with data from another table after insert

Question

I have two unrelated tables tbl_A & tbl_B

tbl_A

+----+---------------------+------+
| id | url                 | slug |
+----+---------------------+------+
|  1 | http://example.com/ | 3qqd |           
|  2 | http://example.com/ | t8af |           
|  3 | http://example.com/ | sjim |           
|  4 | http://example.com/ | awfo |           
|  5 | http://example.com/ | 6myy |           
+----+---------------------+------+

tbl_A description:

+---------------------+---------------------+------+-----+---------------------+----------------+
| Field               | Type                | Null | Key | Default             | Extra          |
+---------------------+---------------------+------+-----+---------------------+----------------+
| id                  | bigint(20) unsigned | NO   | PRI | NULL                | auto_increment |
| url                 | text                | YES  |     | NULL                |                |
| slug                | varchar(255)        | YES  | MUL | NULL                |                |
+---------------------+---------------------+------+-----+---------------------+----------------+

and another table :

tbl_B

+----+---------------------+---------------------+------+           
| ID | user_name           | url                 | slug |
+----+---------------------+---------------------+------+
|  1 | john.reese          | NULL                | NULL |       
+----+---------------------+---------------------+------+

tbl_B description :

+---------------------+---------------------+------+-----+---------------------+----------------+
| Field               | Type                | Null | Key | Default             | Extra          |
+---------------------+---------------------+------+-----+---------------------+----------------+
| ID                  | bigint(20) unsigned | NO   | PRI | NULL                | auto_increment |
| user_name           | varchar(60)         | NO   | MUL |                     |                |
| url                 | text                | YES  |     | NULL                |                |
| slug                | varchar(255)        | YES  | MUL | NULL                |                |
+---------------------+---------------------+------+-----+---------------------+----------------+

tbl_A.id is unrelated to tbl_B.ID.

tbl_B.ID is userID field and gets filled up dnamically when a new user registers. So that tbl_B.ID gets a row inserted automatically as a user register, tbl_B.ID value gets auto incremented.

tbl_A on the other hand already exists with all the details.

What I want to achieve: whenever a new user registers and userID is INSERT into tbl_B.ID, at the same time it should trigger an update of tbl_B.user and tbl_B.slug with the values taken from tbl_A.user and tbl_A.slug.

Outcome : After ID 1 is added

+----+---------------------+---------------------+------+           
| ID | user_name           | url                 | slug |
+----+---------------------+---------------------+------+
|  1 | john.reese          | http://example.com/ | 3qqd |       
+----+---------------------+---------------------+------+

Hope I am able to explain. I was trying to use triggers but got lost, am a newbie with mysql, please bear with me.

drop trigger if exists bi_tbl_B $$
delimiter $$

create trigger bi_tbl_B before insert on tbl_B
for each row begin
   UPDATE tbl_B
     SET url = url +
       (SELECT url
          FROM tbl_A
          WHERE id = NEW.id)
    WHERE ID = NEW.ID;

end;
$$

delimiter ; 

I don't know whether this is even possible or should I try the other way round.

Add a field user_id in tbl_A and AFTER INSERT on tbl_B update tabl_A.user_id column with the userID from tbl_B.ID

I am open to suggestions, if not trigger then procedures.

Answer 1

It is possible to do it, just not the way you are trying. In the before insert trigger you can change the values being inserted by changing the NEW.field_name variables.

drop trigger if exists bi_tbl_B $$
delimiter $$

create trigger bi_tbl_B before insert on tbl_B
for each row begin
   DECLARE v_slug as varchar(255);
   DECLARE v_url as text;
   SELECT url, slug INTO v_url, v_slug FROM tbl_A WHERE id = NEW.id;
   NEW.url=v_url;
   NEW.slug=v_slug;
end;
$$

delimiter ;