The EXISTS and NOT EXISTS operators are empty set checkers.
It doesn't matter which expressions you specify in the SELECT clause of the subquery.
For example, you could also have written the query as follows:
select o.* from offerings o where not exists (select 'x' from registrations r)
Subquery returning a null value is not the same as a subquery returning nothing, that is, the empty set.
SQL> SQL> drop table offerings; Table dropped.-- www .jav a 2s. c o m SQL> create table offerings( 2 course VARCHAR2(6) not null, 3 begindate DATE not null, 4 trainer NUMBER(4) , 5 location VARCHAR2(8)) ; SQL> SQL> insert into offerings values ('SQL',date '1999-04-12',7902,'DALLAS' ); SQL> insert into offerings values ('JSON',date '1999-08-10',7004,'CHICAGO'); SQL> insert into offerings values ('SQL',date '1999-10-04',7001,'SEATTLE'); SQL> insert into offerings values ('SQL',date '1999-12-13',7001,'DALLAS' ); SQL> insert into offerings values ('JAVA',date '1999-12-13',7004,'SEATTLE'); SQL> insert into offerings values ('XML',date '2000-02-03',7001,'DALLAS' ); SQL> insert into offerings values ('JAVA',date '2000-02-01',7011,'DALLAS' ); SQL> insert into offerings values ('PLS',date '2000-09-11',7008,'DALLAS' ); SQL> insert into offerings values ('XML',date '2000-09-18',NULL,'SEATTLE'); SQL> insert into offerings values ('JSON',date '2000-09-27',7902,'DALLAS' ); SQL> insert into offerings values ('ERM',date '2001-01-15',NULL, NULL ); SQL> insert into offerings values ('PRO',date '2001-02-19',NULL,'DALLAS' ); SQL> insert into offerings values ('RSD',date '2001-02-24',7008,'CHICAGO'); SQL> SQL> drop table registrations; Table dropped. SQL> create table registrations( 2 attendee NUMBER(4) not null, 3 course VARCHAR2(6) , 4 begindate DATE not null, 5 evaluation NUMBER(1) check (evaluation in (1,2,3,4,5))) ; SQL> SQL> insert into registrations values (7002,'SQL',date '1999-04-12',4 ); SQL> insert into registrations values (7934,'SQL',date '1999-04-12',5 ); SQL> insert into registrations values (7006,'SQL',date '1999-04-12',4 ); SQL> insert into registrations values (7011,'SQL',date '1999-04-12',2 ); SQL> insert into registrations values (7008,'SQL',date '1999-10-04',NULL); SQL> insert into registrations values (7009,'SQL',date '1999-10-04',3 ); SQL> insert into registrations values (7902,'SQL',date '1999-10-04',4 ); SQL> insert into registrations values (7902,'SQL',date '1999-12-13',NULL); SQL> insert into registrations values (7006,'SQL',date '1999-12-13',NULL); SQL> insert into registrations values (7003,'JSON',date '1999-08-10',4 ); SQL> insert into registrations values (7012,'JSON',date '1999-08-10',4 ); SQL> insert into registrations values (7902,'JSON',date '1999-08-10',5 ); SQL> insert into registrations values (7010,'JSON',date '2000-09-27',5 ); SQL> insert into registrations values (7002,'JAVA',date '1999-12-13',2 ); SQL> insert into registrations values (7007,'JAVA',date '1999-12-13',NULL ); SQL> insert into registrations values (7011,'JAVA',date '1999-12-13',5 ); SQL> insert into registrations values (7008,'JAVA',date '1999-12-13',5 ); SQL> insert into registrations values (7009,'JAVA',date '1999-12-13',4 ); SQL> insert into registrations values (7004,'JAVA',date '2000-02-01',3 ); SQL> insert into registrations values (7008,'JAVA',date '2000-02-01',4 ); SQL> insert into registrations values (7006,'JAVA',date '2000-02-01',5 ); SQL> insert into registrations values (7012,'XML',date '2000-02-03',4 ); SQL> insert into registrations values (7002,'XML',date '2000-02-03',5 ); SQL> insert into registrations values (7004,'PLS',date '2000-09-11',NULL); SQL> insert into registrations values (7002,'PLS',date '2000-09-11',NULL); SQL> insert into registrations values (7011,'PLS',date '2000-09-11',NULL); SQL> SQL> select o.* 2 from offerings o 3 where not exists 4 (select 'x' 5 from registrations r) 6 SQL>