CASE

expressions in oracle is similar to IF ... THEN ... ELSE logic in SQL statements.
Evaluated in Top to bottom approach, If a condition is true, then corresponding THEN clause is executed and execution jumps to the END CASE clause.

syntax:

CASE [ expression ]
   WHEN condition_1 THEN result_1
   WHEN condition_2 THEN result_2
   ...
   WHEN condition_n THEN result_n
   ELSE result
END

Example:

SELECT emp_name, emp_no, dept_no
  (CASE dept_no
     WHEN 10 THEN 'ACCOUNTS'
     WHEN 20 THEN 'HR'
     WHEN 30 THEN 'OPERATIONS'
     WHEN 40 THEN 'SALES'
     ELSE 'UNKNOWN'
   END) DEPARTMENT
FROM EMPLOYEE
ORDER BY emp_name;