How can I handle uniqueness in this situation? - oracle

Question

I have a table like this:
create table my_table
(
type1 varchar2(10 char),
type2 varchar2(10 char)
);
I want to uniqueness like this;
if type1 column has 'GENERIC' value then just type2 column must be unique for the table. for example;
type1 column has 'GENERIC' value and type2 column has 'value_x' then there must not any type2 column value that equals to 'value_x'.
But other uniqueness is looking for both column. I mean it should be unique by type1 and type2 columns.(of course first rule is constant)
I try to make it with trigger;
CREATE OR REPLACE trigger my_trigger
BEFORE INSERT OR UPDATE
ON my_table
FOR EACH ROW
DECLARE
lvn_count NUMBER :=0;
lvn_count2 NUMBER :=0;
errormessage clob;
MUST_ACCUR_ONE EXCEPTION;
-- PRAGMA AUTONOMOUS_TRANSACTION; --without this it gives mutating error but I cant use this because it will conflict on simultaneous connections
BEGIN
IF :NEW.type1 = 'GENERIC' THEN
SELECT count(1) INTO lvn_count FROM my_table
WHERE type2= :NEW.type2;
ELSE
SELECT count(1) INTO lvn_count2 FROM my_table
WHERE type1= :NEW.type1 and type2= :NEW.type2;
END IF;
IF (lvn_count >= 1 or lvn_count2 >= 1) THEN
RAISE MUST_ACCUR_ONE;
END IF;
END;
But it gives mutating error without pragma . I do not want to use it due to conflict on simultaneous connections. (error because I use same table on trigger)
I try to make it with unique index but I cant manage.
CREATE UNIQUE INDEX my_table_unique_ix
ON my_table (case when type1= 'GENERIC' then 'some_logic_here' else type1 end, type2); -- I know it does not make sense but maybe there is something different that I can use in here.
Examples;
**Example 1**
insert into my_table (type1,type2) values ('a','b'); -- its ok no problem
insert into my_table (type1,type2) values ('a','c'); -- its ok no problem
insert into my_table (type1,type2) values ('c','b'); -- its ok no problem
insert into my_table (type1,type2) values ('GENERIC','b'); -- it should be error because b is exist before (i look just second column because first column value is 'GENERIC')
EXAMPLE 2:
insert into my_table (type1,type2) values ('GENERIC','b'); -- its ok no problem
insert into my_table (type1,type2) values ('a','c'); -- its ok no problem
insert into my_table (type1,type2) values ('d','c'); -- its ok no problem
insert into my_table (type1,type2) values ('d','b'); -- it should be error because second column can not be same as the second column value that first column value is 'GENERIC'

Answer 1

What you're trying to do is not really straightforward in Oracle. One possible (although somewhat cumbersome) approach is to use a combination of
an additional materialized view with refresh (on commit)
a windowing function to compute the number of distinct values per group
a windowing function to compute the number of GENERIC rows per group
a check constraint to ensure that either we have only one DISTINCT value or we don't have GENERIC in the same group
This should work:
create materialized view mv_my_table
refresh on commit
as
select
type1,
type2,
count(distinct type1) over (partition by type2) as distinct_type1_cnt,
count(case when type1 = 'GENERIC' then 1 else null end)
over (partition by type2) as generic_cnt
from my_table;
alter table mv_my_table add constraint chk_type1
CHECK (distinct_Type1_cnt = 1 or generic_cnt = 0);
Now, INSERTing a duplicate won't fail immediately, but the subsequent COMMIT will fail because it triggers the materialized view refresh, and that will cause the check constraint to fire.
Disadvantages
duplicate INSERTs won't fail immediately (making debugging more painful)
depending on the size of your table, the MView refresh might slow down COMMITs considerably
Links
For a more detailed discussion of this approach, see AskTom on cross-row constraints

Answer 2

Try it like this:
CREATE TABLE my_table (
type1 VARCHAR2(10 CHAR),
type2 VARCHAR2(10 CHAR),
type1_unique VARCHAR2(10 CHAR) GENERATED ALWAYS AS ( NULLIF(type1, 'GENERIC') ) VIRTUAL
);
ALTER TABLE MY_TABLE ADD (CONSTRAINT my_table_unique_ix UNIQUE (type1_unique, type2) USING INDEX)
Or an index like this should also work:
CREATE UNIQUE INDEX my_table_unique_ix ON MY_TABLE (NULLIF(type1, 'GENERIC'), type2);
Or doing it in your style (you only missed the END):
CREATE UNIQUE INDEX my_table_unique_ix ON my_table (case when type1= 'GENERIC' then null else type1 end, type2);

Answer 3

Unless I'm missing something obvious, the logic in the answer from #Frank Schmitt can also be implemented using a statement level trigger. It is a lot simpler to implement and does not have the disadvantages that Frank mentions.
create or replace TRIGGER my_table_t
AFTER INSERT OR UPDATE OR DELETE
ON my_table
DECLARE
l_dummy NUMBER;
MUST_ACCUR_ONE EXCEPTION;
BEGIN
WITH constraint_violated AS
(
select
type1,
type2,
count(distinct type1) over (partition by type2) as distinct_type1_cnt,
count(case when type1 = 'GENERIC' then 1 else null end)
over (partition by type2) as generic_cnt
from my_table
)
SELECT 1 INTO l_dummy
FROM constraint_violated
WHERE NOT (distinct_type1_cnt = 1 or generic_cnt = 0) FETCH FIRST 1 ROWS ONLY;
RAISE MUST_ACCUR_ONE;
EXCEPTION WHEN NO_DATA_FOUND THEN
NULL;
END;
/