Oracle SQL - Subquery Subquery

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Introduction

The following code introduces the concept of subqueries by starting with an example of the IN operator.

You could execute the following query to get the desired result: 

  
select attendee 
from   registrations 
where  course in ('JAVA','PLS','XML')

You can use subquery to replace the hard-coded list of courses. 

select attendee 
from   registrations 
where  course in (select code 
                  from   courses 
                  where  category = 'BLD');

Oracle first substitutes the subquery between the parentheses with its result-a number of course codes-and then executes the main query.

To retrieve the correct number of emp, you should use SELECT DISTINCT in the main query to eliminate duplicates.

Demo

SQL>
SQL>-- ww w  .  jav a  2  s  .co  m
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>
SQL> drop table courses;

Table dropped.

SQL> create table courses(
  2  code        VARCHAR2(6)  primary key,
  3  description VARCHAR2(30) not null,
  4  category    CHAR(3)      not null,
  5  duration    NUMBER(2)    not null) ;
SQL> insert into courses values('SQL','Introduction to SQL',         'GEN',4);
SQL> insert into courses values('JSON','Oracle for application users','GEN',1);
SQL> insert into courses values('JAVA','Java for Oracle developers',  'BLD',4);
SQL> insert into courses values('PLS','Introduction to PL/SQL',      'BLD',1);
SQL> insert into courses values('XML','XML for Oracle developers',   'BLD',2);
SQL> insert into courses values('ERM','Data modeling with ERM',      'DSG',3);
SQL> insert into courses values('PMT','Process modeling techniques', 'DSG',1);
SQL> insert into courses values('RSD','Relational system design',    'DEF',2);
SQL> insert into courses values('PRO','Prototyping',                 'DSG',5);
SQL> insert into courses values('GEN','System generation',           'DSG',4);
SQL>
SQL>
SQL> select attendee
  2  from   registrations
  3  where  course in (select code
  4                    from   courses
  5                    where  category = 'BLD');

 ATTENDEE
---------
 07002.00
 07007.00
 07011.00
 07008.00
 07009.00
 07004.00
 07008.00
 07006.00
 07012.00
 07002.00
 07004.00

 ATTENDEE
---------
 07002.00
 07011.00

13 rows selected.

SQL>

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