How does one implement IF-THEN-ELSE in a select statement?

The Oracle decode function acts like a procedural statement inside an SQL statement to return different values or columns based on the values of other columns in the select statement.
Some examples:
select decode(sex, 'M', 'Male', 'F', 'Female', 'Unknown')
from employees;

select a, b, decode( abs(a-b), a-b, 'a > b',
0, 'a = b',
'a < b') from tableX;

select decode( GREATEST(A,B), A, 'A is greater OR EQUAL than B', 'B is greater than A')...

select decode( GREATEST(A,B),
A, decode(A, B, 'A NOT GREATER THAN B', 'A GREATER THAN B'),
'A NOT GREATER THAN B')...
Note: The decode function is not ANSI SQL and is rarely implemented in other RDBMS offerings. It is one of the good things about Oracle, but use it sparingly if portability is required.
From Oracle 8i one can also use CASE statements in SQL. Look at this example:
SELECT ename, CASE WHEN sal>1000 THEN 'Over paid' ELSE 'Under paid' END
FROM emp;