The difference between NOT IN and NOT EXISTS

There is a popular misconception that NOT IN and NOT EXISTS are two ways to filter out rows present in one table and not in another table, with both methods being usable interchangeably. Popular wisdom is that the difference between the two is only in terms of performance (being on the basis of whether the larger table is the “outer” or the “inner” in the query) but that the results are the same.

However, we must remember that such an operation being an Anti-Join fails when NULLs are involved. In Oracle a NULL cannot be compared to any other value, not even another NULL. Therefore, a NOT IN operation would fail if the result set being probed returns a NULL. In such a case, the results of a NOT IN query is 0 rows while a NOT EXISTS query would still show the rows present in the one table but not in the other table.

Here is a simple demonstration :


SQL> -- create the two test tables
SQL>
SQL> drop table results_table purge;

Table dropped.

SQL> drop table query_table purge;

Table dropped.

SQL>
SQL> create table results_table
2 as select owner,object_name,object_type from dba_objects where owner in ('HEMANT','DBSNMP','OUTLN');

Table created.

SQL> create table query_table
2 as select owner,object_name,object_type from dba_objects where owner in ('DBSNMP','OUTLN');

Table created.

SQL>
SQL> -- a NOT IN query
SQL> select r.owner, r.object_name from results_table r
2 where r.owner not in (select q.owner from query_table q)
3 order by 1,2;

OWNER OBJECT_NAME
------------------------------ ------------------------------
HEMANT DUPDB
HEMANT MY_T_A
HEMANT RESULTS_TABLE
HEMANT SOURCE_TABLE
HEMANT TEST_APPEND
HEMANT TRACE_USER_SESSIONS
HEMANT TRACE_USER_SESSIONS_BEGIN
HEMANT TRACE_USER_SESSIONS_END

8 rows selected.

SQL>
SQL> -- a NOT EXISTS query
SQL> select r.owner, r.object_name from results_table r
2 where not exists (select '1' from query_table q where r.owner=q.owner)
3 order by 1,2;

OWNER OBJECT_NAME
------------------------------ ------------------------------
HEMANT DUPDB
HEMANT MY_T_A
HEMANT RESULTS_TABLE
HEMANT SOURCE_TABLE
HEMANT TEST_APPEND
HEMANT TRACE_USER_SESSIONS
HEMANT TRACE_USER_SESSIONS_BEGIN
HEMANT TRACE_USER_SESSIONS_END 



8 rows selected.

SQL>
SQL> ---
SQL> REM So far, NOT IN and NOT EXISTS have presented the same results
SQL>
SQL> REM What happens if there is a row with a NULL value ?
SQL>
SQL> insert into query_table values (NULL,'ABCDEFGH','TABLE');

1 row created.

SQL> commit;

Commit complete.

SQL>
SQL> -- retry the NOT IN query
SQL> select r.owner, r.object_name from results_table r
2 where r.owner not in (select q.owner from query_table q)
3 order by 1,2;

no rows selected

SQL>
SQL> -- retry the NOT EXISTS query
SQL> select r.owner, r.object_name from results_table r
2 where not exists (select '1' from query_table q where r.owner=q.owner)
3 order by 1,2;

OWNER                          OBJECT_NAME                                                                                         
------------------------------ ------------------------------
HEMANT DUPDB
HEMANT MY_T_A
HEMANT RESULTS_TABLE
HEMANT SOURCE_TABLE
HEMANT TEST_APPEND
HEMANT TRACE_USER_SESSIONS
HEMANT TRACE_USER_SESSIONS_BEGIN
HEMANT TRACE_USER_SESSIONS_END

8 rows selected.

SQL>
SQL> ---
SQL> REM Surprise ?! The NOT IN returned 0 rows !
SQL> REM Why ? Because of the presence of a NULL in the query_table !
SQL> REM
SQL> REM REMEMBER : A "NOT IN" anti-join fails because a NULL returned cannot be compared !
SQL>
SQL> --
SQL> REM One "workaround" is to filter out rows which contain NULLs
SQL> REM .... but think carefully before you do so. Are you sure you want to exclude them ?
SQL>
SQL> REM In the ideal world, such columns should be defined as NOT NULL columns !
SQL> REM That would be the right schema design !
SQL>
SQL> -- test the suggested workaround
SQL> select r.owner, r.object_name from results_table r
2 where r.owner not in (select q.owner from query_table q WHERE OWNER IS NOT NULL)
3 order by 1,2;


OWNER                          OBJECT_NAME                                                                                         
------------------------------ ------------------------------
HEMANT DUPDB
HEMANT MY_T_A
HEMANT RESULTS_TABLE
HEMANT SOURCE_TABLE
HEMANT TEST_APPEND
HEMANT TRACE_USER_SESSIONS
HEMANT TRACE_USER_SESSIONS_BEGIN
HEMANT TRACE_USER_SESSIONS_END

8 rows selected.

SQL> 
 
 
 
Finally I would thank Hemant for this amazing Article .
I posted to make it more popular and useful for the people who wants to learn
something new .
 
Osama mustafa  

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