Explain CASE expressions and DECODE

A CASE expression returns a value: simple CASE x WHEN 1 THEN ... , or searched CASE WHEN x > 0 THEN ... ELSE ... END. It is distinct from the CASE statement (which ends with END CASE). DECODE(expr, s1, r1, s2, r2, default) does the same by equality only, treats two NULLs as equal (CASE does not), and is SQL-only. Prefer CASE for readability and portability; reach for DECODE mainly in legacy code.

SELECT last_name,
       CASE
         WHEN salary >= 15000 THEN 'A'
         WHEN salary >= 8000  THEN 'B'
         ELSE 'C'
       END AS band,
       DECODE(department_id, 10, 'Admin', 20, 'Mktg', 'Other') AS dept_label
  FROM employees;