mysql trigger after insert not working - mysql

Question

I tried from several examples and could not reach my goal - so I am asking here...
I have a table :
table1
columns : col1 , col2
I want to change col2 after an insert :
If (col2 = 0) then
alter col2 to be max(col2) + 1
else
leave the value as it is
Thanks

Answer 1

You need to do it using before insert since before insert you can check if the col2 is 0 and then set the value to max col2 + 1
Here is a way to do it
delimiter //
create trigger test_ins before insert on table1
for each row
begin
declare max_col2 int;
if new.col2 = 0 then
set max_col2 = (select max(col2) from table1);
end if ;
if max_col2 is null then
set new.col2 = 1 ;
else
set new.col2 = max_col2 + 1 ;
end if ;
end ;//
delimiter ;
Note that in the above trigger I have added a check such that if the max(col2) is null which will happen when you add the first record in the table and col2 as 0 I am setting it as 1
if max_col2 is null then
set new.col2 = 1 ;
You can set it as you want.
Here is a test case
mysql> create table table1 (col1 int, col2 int);
Query OK, 0 rows affected (0.13 sec)
mysql> delimiter //
mysql> create trigger test_ins before insert on table1
-> for each row
-> begin
-> declare max_col2 int;
-> if new.col2 = 0 then
-> set max_col2 = (select max(col2) from table1);
-> end if ;
-> if max_col2 is null then
-> set new.col2 = 1 ;
-> else
-> set new.col2 = max_col2 + 1 ;
-> end if ;
-> end ;//
Query OK, 0 rows affected (0.10 sec)
mysql> delimiter ;
mysql> insert into table1 values (1,0);
Query OK, 1 row affected (0.04 sec)
mysql> select * from table1;
+------+------+
| col1 | col2 |
+------+------+
| 1 | 1 |
+------+------+
1 row in set (0.00 sec)
mysql> insert into table1 values (2,0);
Query OK, 1 row affected (0.04 sec)
mysql> select * from table1;
+------+------+
| col1 | col2 |
+------+------+
| 1 | 1 |
| 2 | 2 |
+------+------+
2 rows in set (0.00 sec)

Answer 2

Again - thanks for your reply.
However, I tested it and it seems :
Not to add + 1 to the next value (and I really do not know why it is like that).
Leave the value if it is defined (I wrote above: "else leave the value as it is")
I changed your code a bit and put some extra IFs.
Now it is working OK (for me).
DELIMITER //
CREATE TRIGGER trigCF7Submits
BEFORE INSERT ON wp_cf7dbplugin_submits
FOR EACH ROW
BEGIN
DECLARE nMaxInternalNumber INT;
IF ((new.form_name='ActionForm') AND (new.field_name='txtActionFormInternalNumber')) then
IF (new.field_value = 0) THEN
SET nMaxInternalNumber = (SELECT max(field_value) FROM wp_cf7dbplugin_submits WHERE ((form_name='ActionForm') AND (field_name='txtActionFormInternalNumber')));
IF (nMaxInternalNumber IS NULL) THEN
SET nMaxInternalNumber = 0;
END IF;
SET nMaxInternalNumber = nMaxInternalNumber + 1;
ELSE
SET nMaxInternalNumber = new.field_value;
END IF;
SET new.field_value = nMaxInternalNumber ;
END IF ;
END ;//
DELIMITER ;