Single-row subqueries return only one row of result.

    
--A single-row subquery uses a single-row operator; the common operator is the equality operator (=). 

SQL>
SQL>
SQL> CREATE TABLE EMP(
  2      EMPNO NUMBER(4) NOT NULL,
  3      ENAME VARCHAR2(10),
  4      JOB VARCHAR2(9),
  5      MGR NUMBER(4),
  6      HIREDATE DATE,
  7      SAL NUMBER(7, 2),
  8      COMM NUMBER(7, 2),
  9      DEPTNO NUMBER(2)
 10  );

Table created.




SQL> INSERT INTO EMP VALUES(2, 'Jack', 'Tester', 6,TO_DATE('20-FEB-1981', 'DD-MON-YYYY'), 1600, 300, 30);

1 row created.

SQL> INSERT INTO EMP VALUES(3, 'Wil', 'Tester', 6,TO_DATE('22-FEB-1981', 'DD-MON-YYYY'), 1250, 500, 30);

1 row created.

SQL> INSERT INTO EMP VALUES(4, 'Jane', 'Designer', 9,TO_DATE('2-APR-1981', 'DD-MON-YYYY'), 2975, NULL, 20);

1 row created.

SQL> INSERT INTO EMP VALUES(5, 'Mary', 'Tester', 6,TO_DATE('28-SEP-1981', 'DD-MON-YYYY'), 1250, 1400, 30);

1 row created.

SQL> INSERT INTO EMP VALUES(6, 'Black', 'Designer', 9,TO_DATE('1-MAY-1981', 'DD-MON-YYYY'), 2850, NULL, 30);

1 row created.

SQL> INSERT INTO EMP VALUES(7, 'Chris', 'Designer', 9,TO_DATE('9-JUN-1981', 'DD-MON-YYYY'), 2450, NULL, 10);

1 row created.

SQL> INSERT INTO EMP VALUES(8, 'Smart', 'Helper', 4,TO_DATE('09-DEC-1982', 'DD-MON-YYYY'), 3000, NULL, 20);

1 row created.

SQL> INSERT INTO EMP VALUES(9, 'Peter', 'Manager', NULL,TO_DATE('17-NOV-1981', 'DD-MON-YYYY'), 5000, NULL, 10);

1 row created.

SQL> INSERT INTO EMP VALUES(10, 'Take', 'Tester', 6,TO_DATE('8-SEP-1981', 'DD-MON-YYYY'), 1500, 0, 30);

1 row created.



SQL> INSERT INTO EMP VALUES(13, 'Fake', 'Helper', 4,TO_DATE('3-DEC-1981', 'DD-MON-YYYY'), 3000, NULL, 20);

1 row created.

SQL>
SQL>
SQL> SELECT ename, sal
  2      FROM   EMP
  3      WHERE  sal= (SELECT MAX(sal) FROM emp);

ENAME             SAL
---------- ----------
Peter            5000

SQL>
SQL>
SQL> drop table emp;

Table dropped.

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Single-row subqueries return only one row of result. : Subquery Basics « Subquery « Oracle PL / SQL

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