For Access SQL use either the IIF( <condition>, <true expression>, <false expression> ), which can be nested (that is the condition or the true/false expresssions can also be IIF's), or use the Choose( <number>, <expression when 1>, <expression when 2>, ..., <expression when n> ) function to return an expression from the value of the number.
If using VBA try the Select Case ... End Case statement.