nested if , and how to trigger....

Hi,
can we do nested if in function ? would you give me some example?

second, if i've create table like this:

create table tb (
code varchar(4),
classes varchar(6),
lecture varchar(4),
th int2,
day varchar(1),
hr varchar(1),
room varchar(4),
primary key (code,kl,ds));

my questions is how to prevent this record to be insert:
code classes lecture th day hr room
K021 1tip01 1011 5 1 1 1111
K022 1tip01 1011 5 1 1 1111

or :
code classes lecture th day hr room
K021 1tip01 1011 5 1 1 1111
K021 1tip01 1012 5 1 1 1111

TIA
Didi