NOT IN sql querry

Question

Hi
I had two tables
--USER LEVEL CATALOG TABLE
create table user_cat
(user_id number,
catloag_name varchar2(40),
catalog_id   number,
item_id      number );
Insert into USER_CAT (USER_ID, CATALOG_NAME, CATALOG_ID, ITEM_ID)
Values (1, 'DELL', 12, 1001);
Insert into USER_CAT (USER_ID, CATALOG_NAME, CATALOG_ID, ITEM_ID)
Values (1, 'DELL', 12, 1002);
Insert into USER_CAT (USER_ID, CATALOG_NAME, CATALOG_ID, ITEM_ID)
Values (1, 'NOKIA', 13, 1003);
Insert into USER_CAT (USER_ID, CATALOG_NAME, CATALOG_ID, ITEM_ID)
Values (1, 'NOKIA', 13, 1004);
Insert into USER_CAT (USER_ID, CATALOG_NAME, CATALOG_ID, ITEM_ID)
Values (2, 'DELL', 12, 1001);
Insert into USER_CAT (USER_ID, CATALOG_NAME, CATALOG_ID, ITEM_ID)
Values (2, 'DELL', 12, 1010);
Insert into USER_CAT (USER_ID, CATALOG_NAME, CATALOG_ID, ITEM_ID)
Values (2, 'HP', 11, 1011);
Insert into USER_CAT (USER_ID, CATALOG_NAME, CATALOG_ID, ITEM_ID)
Values (2, 'HP', 11, 1012);
Insert into USER_CAT (USER_ID, CATALOG_NAME, CATALOG_ID, ITEM_ID)
Values (2, 'HP', 11, 1014);
COMMIT;
--Catalog Level  Exclusion Table  which should not contains the USER_ID
--This table contains the records needs to be excluded .
create table cat_exclusion
( catalog_nam varchar2(40),
  catalog_id number ,
item_id  number);
SET DEFINE OFF;
Insert into CAT_EXCLUSION
   (CATALOG_NAME, CATALOG_ID, ITEM_ID)
Values
   ('DELL', 12, 1001);
Insert into CAT_EXCLUSION
   (CATALOG_NAME, CATALOG_ID, ITEM_ID)
Values
   ('NOKIA', 13, 1004);
Insert into CAT_EXCLUSION
   (CATALOG_NAME, CATALOG_ID, ITEM_ID)
Values
   ('NOKIA', 13, 1003);
COMMIT;
Now in the CAT_EXCLUSION table  I excluded the below records
For USER 1
-- DELL   12 1001
-- NOKIA 13 1004
AND for user 2 I excluded
--NOKIA 13 1003
Now i want to get the all other records for USER_ID =2 except for the excluded records.
I tried to do this using NOT IN below as shown but it was also excluding the other values.Please let me know how can we do this ?
Select   USER_ID
       , CATALOG_NAME
       , CATALOG_ID
       , ITEM_ID
  From   user_cat
Where   item_id Not In (Select   item_id From cat_exclusion)
    And  user_id = 2

Answer 1

You have to be careful about using NOT IN (SELECT...)
If one of the rows in the subquery is a null, you won't get any rows at all:
SQL> create table t as select * from dba_objects;
Table created.
SQL> create table s (object_type varchar2(30));
Table created.
SQL> insert into s values ('TABLE');
1 row created.
SQL> insert into s values ('INDEX');
1 row created.
SQL> insert into s values (null);
1 row created.
SQL> commit;
Commit complete.
SQL> select count(*)
  2  from   t
  3  where  object_type not in (select object_type from s);
            COUNT(*)
                   0
SQL> select count(*)
  2  from   t
  3  where  object_type not in (select object_type from s WHERE OBJECT_TYPE IS NOT NULL);
            COUNT(*)
               69505