if else statement

[Song Ken Vern-E11804]

Hi,
 
I'm trying to build a query in using SQL instead of doing it in Perl. 

I am trying to do something like this : 

If ((select col1 from table1 where id = 1) == 3)
Then 
Select col2 from table2 where table2.id = 1;
Else
Select col2 from table3 where table3.id = 1;

In Perl I would probably do have to access the DB twice. 

Select col2 from table1 where if = 1; 

If (col2 == 3) { 
 Select col2 from table2 where table2.id = 1;
} else { 
 Select col2 from table3 where table3.id = 1;
}

I've read the manual on subqueries but the example don't indicate how I
can 
do a conditional test using a subquery?

Am I on the right track or is there another way to do this?

Thanks

--
Ken
e11804