Oracle SQL - EXISTS, IN, or JOIN?

Introduction

The following query retrieves the personal details of all emp who ever taught an SQL course.

Correlated Subquery with EXISTS Operator

select e.*
from   emp e
where  exists (select o.*
               from   offerings o
               where  o.course = 'SQL'
               and    o.trainer = e.empno);

You can also use an IN operator.

select e.*
from   emp e
where  e.empno in (select o.trainer
                   from   offerings o
                   where  o.course = 'SQL')

You can also use a join to solve the problem.

select DISTINCT e.*
from   emp e
       join
       offerings o
       on e.empno = o.trainer
where  o.course = 'SQL'

Demo

SQL>
SQL> drop table emp;

Table dropped.--   w  w  w  . jav a 2 s .  c o m

SQL> create table emp(
  2  empno      NUMBER(4)    primary key,
  3  ename      VARCHAR2(8)  not null   ,
  4  init       VARCHAR2(5)  not null   ,
  5  job        VARCHAR2(8)             ,
  6  mgr        NUMBER(4)               ,
  7  bdate      DATE         not null   ,
  8  msal       NUMBER(6,2)  not null   ,
  9  comm       NUMBER(6,2)             ,
 10  deptno     NUMBER(2)    default 10) ;
SQL>
SQL> insert into emp values(7001,'SMITH','N',  'TRAINER', 7902,date '1975-12-17',  1800 , NULL, 20);
SQL> insert into emp values(7002,'ALLEN','JAM','SALESREP',7006,date '1971-05-20',  1600, 300,   30);
SQL> insert into emp values(7003,'WARD', 'TF' ,'SALESREP',7006,date '1972-03-02',  1250, 500,   10);
SQL> insert into emp values(7004,'JACK', 'JM', 'MANAGER', 7009,date '1977-04-02',  2975, NULL,  20);
SQL> insert into emp values(7005,'BROWN','P',  'SALESREP',7006,date '1976-09-28',  1250, 1400,  30);
SQL> insert into emp values(7006,'BLAKE','R',  'MANAGER', 7009,date '1973-11-01',  2850, NULL,  10);
SQL> insert into emp values(7007,'CLARK','AB', 'MANAGER', 7009,date '1975-06-09',  2450, NULL,  10);
SQL> insert into emp values(7008,'SCOTT','DEF','TRAINER', 7004,date '1979-11-26',  3000, NULL,  20);
SQL> insert into emp values(7009,'KING', 'CC', 'DIRECTOR',NULL,date '1972-10-17',  5000, NULL,  10);
SQL> insert into emp values(7010,'BREAD','JJ', 'SALESREP',7006,date '1978-09-28',  1500, 0,     30);
SQL> insert into emp values(7011,'ADAMS','AA', 'TRAINER', 7008,date '1976-12-30',  1100, NULL,  20);
SQL> insert into emp values(7012,'JONES','R',  'ADMIN',   7006,date '1979-10-03',  8000, NULL,  30);
SQL> insert into emp values(7902,'FORD', 'MG', 'TRAINER', 7004,date '1979-02-13',  3000, NULL,  20);
SQL> insert into emp values(7934,'MARY', 'ABC','ADMIN',   7007,date '1972-01-23',  1300, NULL,  10);
SQL>
SQL> drop table offerings;

Table dropped.

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> select e.*
  2  from   emp e
  3  where  exists (select o.*
  4                 from   offerings o
  5                 where  o.course = 'SQL'
  6                 and    o.trainer = e.empno);

    EMPNO | ENAME    | INIT  | JOB      |       MGR | BDATE      |      MSAL
--------- | -------- | ----- | -------- | --------- | ---------- | ---------
     COMM |    DEPTNO
--------- | ---------
 07001.00 | SMITH    | N     | TRAINER  |  07902.00 | 17-12-1975 |  01800.00
 [N/A]    |  00020.00

 07902.00 | FORD     | MG               |  07004.00 | 13-02-1979 |  03000.00
 [N/A]    |  00020.00


SQL>
SQL>
SQL> select e.*
  2  from   emp e
  3  where  e.empno in (select o.trainer
  4                     from   offerings o
  5                     where  o.course = 'SQL')
  6
SQL>
SQL> select DISTINCT e.*
  2  from   emp e
  3         join
  4         offerings o
  5         on e.empno = o.trainer
  6  where  o.course = 'SQL'
  7

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