1 -- nvl
 2 select nvl('1','2') from dual;
 3 
 4 select nvl(null,'2') from dual;
 5 
 6 -- nvl2
 7 select nvl2('1','2','3') from dual;
 8 
 9 select nvl2(null,'2','3') from dual;
10 
11 -- coalesce
12 select coalesce('是',null,'否') from dual;
13 
14 select coalesce(null,'是','否') from dual;
15 
16 select coalesce(null,null,'否') from dual;
17 
18 select coalesce(null,null,null) from dual;