create columns using the row value of another column

[usuario_estudo]

Table Have

ID	ID_REF	X
1	4	0,33
2	4	0,4
3	4	0,12
4	4	0,8
5	4	0,154

Table Want 

ID	ID_REF	X	Y
1	4	0,33	0,33
2	4	0,4	0,4
3	4	0,12	0,12
4	4	0,8	0,8
5	4	0,154	0,8

I want to create a code to create the column Y, whose value will be:

when ID<ID_REF, then Y=X,
when ID>=ID_REF, then Y will always have the same value, which will be the value of X when ID=ID_REF

[novinosrin · Accepted solution]

data want;
 set have;
 by id_ref;
 retain y;
 if first.id_ref then call missing(y);
 if ID<=ID_REF then y=x;
run;

[PGStats]

I would do:

data want;
retain y;
set have;
if id <= id_ref then y = x;
output;
if id < id_ref then call missing (y);
run;