Dynamic Trigger to Search and Update Table Record

This would be failry simple something as

delimiter //
  create trigger number_clicks after insert on event
  for each row
    begin
    declare click_count int default 0 ;
    select count(*) 
    into click_count
    from clicks 
    where eventId = new.eventId ;

    if click_count > 0 then  
       update clicks set numberClicks = numberClicks + 1 where eventId = new.eventId ;
    else
       insert into clicks (eventId , numberClicks)  values (new.eventId,1);
    end if;
    end ;//  
delimiter ;

So the idea is first check in the click table if there is an entry for the last inserted event and if its there then increment the count by one else add a new entry in the table

Here are some tests

mysql> create table event (eventId int,userId int);
Query OK, 0 rows affected (0.06 sec)

mysql> create table clicks(eventId int , numberClicks int);
Query OK, 0 rows affected (0.08 sec)

mysql> insert into event values (1,1);
Query OK, 1 row affected (0.00 sec)

mysql> select * from clicks ;
+---------+--------------+
| eventId | numberClicks |
+---------+--------------+
|       1 |            1 |
+---------+--------------+
1 row in set (0.00 sec)

mysql> insert into event values (1,2);
Query OK, 1 row affected (0.00 sec)

mysql> select * from clicks ;
+---------+--------------+
| eventId | numberClicks |
+---------+--------------+
|       1 |            2 |
+---------+--------------+
1 row in set (0.00 sec)

mysql> insert into event values (2,4);
Query OK, 1 row affected (0.00 sec)

mysql> select * from clicks ;
+---------+--------------+
| eventId | numberClicks |
+---------+--------------+
|       1 |            2 |
|       2 |            1 |
+---------+--------------+
2 rows in set (0.00 sec)

mysql> insert into event values (2,5);
Query OK, 1 row affected (0.00 sec)

mysql> select * from clicks ;
+---------+--------------+
| eventId | numberClicks |
+---------+--------------+
|       1 |            2 |
|       2 |            2 |
+---------+--------------+
2 rows in set (0.00 sec)