create table t1 (id number,x_ varchar2( 20 ));
create table t2 (x_ varchar2( 20 ),y_ varchar2( 20 ));
insert into t1 values ( 1 ,'x1');
insert into t1 values ( 2 ,'x2');
insert into t1 values ( 3 ,'x3');
insert into t1 values ( 4 ,'x4');
insert into t1 values ( 5 ,'x5');
insert into t1 values ( 6 ,'x2');
insert into t1 values ( 7 ,'x3');

insert into t2 values ('x1','y1');
insert into t2 values ('x2','y2');
insert into t2 values ('x3','y3');
insert into t2 values ('x4','y4');

select m.id,nvl(r.y_,m.x_) from t1 m,t2 r where m.x_=r.x_(+)