Oracle trigger to prevent specified value wrote into column - oracle11g

Question

I'm working on Oracle 11g 64bit.
Say I have a table named "MyTable", I'm trying to monitoring a column named "My_Name".
When "My_Name" is going to be changed to '' (before update), I want to stop it and change "My_Name" back to old value. In other word, '' isn't a legal value for the "My_Name" column.
Here's what I did so far, no compilation error, but no effect, I can still write the '' value into "My_Name" column.
CREATE OR REPLACE TRIGGER MyTable_tracking
BEFORE INSERT OR UPDATE ON MyDB.MyTable REFERENCING NEW AS newValue OLD AS oldValue
FOR EACH ROW
DECLARE
v_old VARCHAR(20);
v_new VARCHAR(20);
BEGIN
IF INSERTING THEN
v_new:=:newValue.My_Name; --Trigger checks column 'My_Name' only
ELSIF UPDATING THEN
v_old:=:oldValue.My_Name; --Trigger checks column 'My_Name' only
v_new:=:newValue.My_Name; --Trigger checks column 'My_Name' only
--IF :newValue.My_Name='' THEN
IF LENGTH(TRIM(:newValue.My_Name))=0 THEN
:newValue.My_Name:=:oldValue.My_Name;
END IF;
END IF;
END;
How can I do this?

Answer 1

No need for a trigger. In Oracle an empty string '' and null are the same thing. So just define my_name as NOT NULL and you can't put null or '' in it.
SQL> create table my_table (id integer primary key, my_name varchar(20) not null);
Table created.
SQL> insert into my_table values (1, 'Arthur');
1 row created.
SQL> update my_table set my_name = '' where id = 1;
update my_table set my_name = '' where id = 1
*
ERROR at line 1:
ORA-01407: cannot update ("ARTHUR"."MY_TABLE"."MY_NAME") to NULL
SQL> insert into my_table values (2, '');
insert into my_table values (2, '')
*
ERROR at line 1:
ORA-01400: cannot insert NULL into ("ARTHUR"."MY_TABLE"."MY_NAME")
SQL>
If you can't use the most efficient solution because of external restrictions (which I find highly questionable), you can use something like this:
create or replace trigger slow_not_null_check
before insert or update on my_table
for each row
begin
if inserting and :new.my_name is null then
:new.my_name := 'No NULL allowed';
end if;
if updating and :new.my_name is null then
:new.my_name := :old.my_name;
end if;
end;
/
This will silently convert, '' to 'No NULL allowed' when inserting and will restore the previous value when updating:
insert into my_table values (1, '');
insert into my_table values (2, 'Arthur');
select * from my_table;
ID | MY_NAME
---+----------------
1 | No NULL allowed
2 | Arthur
update my_table
set my_name = ''
where id = 2;
select *
from my_table;
ID | MY_NAME
---+----------------
1 | No NULL allowed
2 | Arthur

Answer 2

I fixed my problem whit following code
IF INSERTING THEN
v_new:=:newValue.My_Name;
ELSIF UPDATING THEN
v_old:=:oldValue.My_Name;
v_new:=:newValue.My_Name;
--IF :newValue.My_Name='some specified value' THEN --not allow some value
--IF :newValue.My_Name='' --not working
IF :newValue.My_Name='' OR :newValue.My_Name IS NULL THEN --not allow '' or null
:newValue.My_Name:=v_old; --works
--:newValue.My_Name:=:oldValue.My_Name; --not working, use variable instead
END IF;
END IF;