--drop table t_A;
--drop table t_B;
create temp table t_A ( f1 int ) with no log;  
create temp table t_B ( f1 int ) with no log;  
insert into t_A values( 1 );
insert into t_B values( 1 );
select * from t_B
  where ( CASE WHEN f1 IN ( select f1 from t_A )
            THEN  2 
            ELSE  3  END ) =  2 ;
 f1    
 ----- 
  1      

  1  record(s) selected 

insert into t_A values( 1 );
select * from t_B
  where ( CASE WHEN f1 IN ( select f1 from t_A )
            THEN  2 
            ELSE  3  END ) =  2 ; -- ??
 f1    
 ----- 

  0  record(s) selected

select * from t_B
  where ( CASE WHEN f1 IN (  1 ,  1  )
            THEN  2 
            ELSE  3  END ) =  2 ;
 f1    
 ----- 
  1      

  1  record(s) selected