How do I perform an IF…THEN in an SQL SELECT?

The CASE statement is the closest to IF in SQL and is supported on all versions of SQL Server:

SELECT CAST( 
   CASE
      WHEN
         Obsolete = 'N' 
         or InStock = 'Y' 
      THEN 1 
      ELSE 0 
   END AS bit) as Saleable, * 
FROM
   Product