Trigger created with compiling errors on unique value - sql

Question

I'm trying to create a trigger that checks the value of a column whenever I have an insert or an update on the table, the value of columnX must be unique:
tableX(ID, ..., columnX)
CREATE or replace TRIGGER tableX_uk
BEFORE INSERT OR UPDATE ON tableX
FOR EACH ROW
BEGIN
if(:new.columnX in (select T.columnX from tableX T)) then
Raise_Application_Error(-20001, 'Already existing');
end if;
End;
It shows that the trigger is created with compiling errors.
I couldn't find any error here, can someone help me please ? Thank you !

Answer 1

It shows that the trigger is created with compiling errors. I couldn't find any error here
Errors for stored PL/SQL can be found in user/all/dba_errors. Desktop tools such as SQL Developer, Toad, PL/SQL Developer etc will display them automatically but that's where they get the details from.
DBFiddle
In this case the first error is from
if(:new.columnX in (select t.columnX from tableX t))
which gives
PLS-00405: subquery not allowed in this context
because if x in (select...) isn't valid PL/SQL syntax. You have to do the select as a separate step. Fixing that will give you code that at least compiles, but still isn't ideal:
create or replace trigger tablex_uk_trg
before insert or update on tablex
for each row
declare
l_columnx_check number := 0;
begin
select count(*) into l_columnx_check
from tablex t
where t.columnx = :new.columnx
and rownum = 1;
if l_columnx_check > 0 then
raise_application_error(-20001, 'Value '||:new.columnx||' already exists');
end if;
end;
It's not ideal because firstly, a unique constraint is a far more efficient and self-documenting way to enforce uniqueness.
create table tablex (columnx number unique);
or better still
create table tablex (columnx number constraint tablex_uk unique);
or if it's the primary key
create table tablex (columnx number constraint tablex_pk primary key);
Now, anyone checking the table definition will see the unique constraint, the optimiser will use it in queries, it has a standard error code ORA-00001: unique constraint (WILLIAM.TABLEX_UK) violated and so on.
Secondly, the update part of the trigger won't work anyway. Oracle won't let a row-level update trigger query its own table:
insert into tablex (columnx) values (1);
update tablex set columnx = 1;
ORA-04091: table WILLIAM.TABLEX is mutating, trigger/function may not see it
ORA-06512: at "WILLIAM.TABLEX_UK_TRG", line 4
ORA-04088: error during execution of trigger 'WILLIAM.TABLEX_UK'
DBFiddle
And in any case the logic is missing some checks, because the update in my example should be valid. But I won't go into how you might fix that because the unique constraint is all you need.