case()的用法与decode()基本相同：
语法格式为：
selectename ,caseename
when'KING'then'Hello'
when'FORD'then'Word'
when'MILLER'then'HELLOWORD'
elseename
end name
fromemp
wheredeptno =10
ENAME NAME
---------- ----------
CLARK CLARK
KING Hello
MILLER HELLOWORD