select * from s;

     ID            Y
======= ============
      1          200
      2          400

select * from t;

     ID            X
======= ============
      1          100

SQL> merge into t using (select * from s where id>=2) s on s.id=t.id 
CON> when not matched then insert values(s.id, s.y) 
CON> returning new.id as new_t_id, new.x as new_t_x;

    NEW_T_ID      NEW_T_X
============ ============
           2          400

SQL> merge into t using s on s.id = t.id when not matched then insert values(s.id, s.y) returning new.id as new_t_id, new.x as new_t_x;

Statement failed, SQLSTATE = 21000
multiple rows in singleton select

Statement 25:
-------------------------------------------------------------------------------
merge into t using s on s.id=t.id when not matched then insert values(s.id, s.y)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
PLAN JOIN (S NATURAL, T INDEX (RDB$PRIMARY3))
0 records fetched
      0 ms, 1 read(s), 19 fetch(es), 2 mark(s)

Table                             Natural     Index    Update    Insert    Delete
**********************************************************************************
 T                                                 1                   1 
 S                                       2