SQL> select * from test;

       ENO SAL
---------- ----------
         1 low
         2 medium
         3 high
         3 high
         1 low
        10 high
        10 low
         7 HIGH

8 rows selected.

SQL> select eno,decode(sal,'HIGH',1,'medium',2,'low',3, 0) test,sal from test;

       ENO       TEST SAL
---------- ---------- ----------
         1          0 low
         2          0 medium
         3          0 high
         3          0 high
         1          0 low
        10          0 high
        10          0 low
         7          0 HIGH

8 rows selected.

SQL> create table test (eno varchar2(2), sal varchar2(10));

Table created.

SQL> insert all
  2    into test values ('N1', 'Low')
  3    into test values ('N2', 'Medium')
  4    into test values ('N3', 'High')
  5  select * From dual;

3 rows created.

SQL> select * From test;

EN SAL
-- ----------
N1 Low
N2 Medium
N3 High

SQL> select sal
  2  from test
  3  order by eno desc;

SAL
----------
High
Medium
Low

SQL> select sal
  2  from test
  3  order by decode(sal, 'High', 1,
  4                       'Medium', 2,
  5                       'Low', 3
  6                 );

SAL
----------
High
Medium
Low

SQL>  select length(sal) len , sal from test;

       LEN  SAL
-------------------------------------------
        10 low

        10 medium

        10 high
      .
      .
      .
8 rows selected.

SQL> desc test;
 Name                                                  Null?    Type
 ----------------------------------------------------- -------- 
 ENO                                                            NUMBER
 SAL                                                            VARCHAR2(100)

SQL>  select eno,decode(trim(sal),'high',1,'medium',2,'low',3, 0) test,sal
 from test order by decode(trim(sal),'high',1,'medium',2,'low',3);

       ENO       TEST SAL
---------- ---------- ----------
         3          1 high
        10          1 high
         3          1 high
         2          2 medium
        10          3 low
         1          3 low
         1          3 low

7 rows selected.