Updating lookup table using merge query

Question

Hi Experts,
My requirement is, we have total 4 tables called x,y,z and a_lookup table. join column between all these tables is deptno.
I need to update a_lookup table based on below conditions
   condition1 :   Update a_lookup table with matched records of X and Y
   condition2:    If there is any record in X is not matching with Y, then we need to compare with Z and update the a_lookup table accordingly
Here is the table scripts and my attempt as well.
Only doubt is,  is my MERGE statement looks fine or is there any other better way to update the a_lookup table ?
Please share your thoughts on this.
create table x(empno number, deptno number);
  -- sample data
  insert into x
    select level, level * 10 from dual connect by level <= 10;
  create table y(empno number, deptno number);
  -- sample data
  insert into y
    select level, level * 10 from dual connect by level <= 5;
  create table z(empno number, deptno number);
  -- sample data
  insert into z
    select level, level * 10 from dual connect by level <= 10;
  create table a_lookup(empno number, deptno_lookup number);
  -- sample data
  insert into a_lookup
    select null, level * 10 from dual connect by level <= 10;
  -- Merging records into a_lookup based on X,Y,Z. Used right outer join on X and Y
merge into a_lookup a
using ( (select  x.deptno,x.empno   from x,y where x.deptno=y.deptno)
           union all
           (select z.deptno,z.empno from z, (select x.deptno from x,y where x.deptno=y.deptno and y.deptno is null)  res1
              where z.deptno = res1.deptno)
           ) res
on( a.deptno_lookup = res.deptno)
when matched then
  update set a.empno = res.empno;
Cheers,
Suri ;-)

Answer 1

Assuming empno is unique in X, Y and Z:
merge
  into a_lookup a
  using (
         select  nvl(y.empno,z.empno) empno,
                 x.deptno
           from  x,
                 y,
                 z
           where y.deptno(+) = x.deptno
             and z.deptno(+) = x.deptno
        ) b
  on (
          a.deptno_lookup = b.deptno
      and
          b.empno is not null
  when matched
    then
      update
         set a.empno = b.empno
10 rows merged.
SCOTT@orcl > select * from a_lookup;
     EMPNO DEPTNO_LOOKUP
         1            10
         2            20
         3            30
         4            40
         5            50
         6            60
         7            70
         8            80
         9            90
        10           100
10 rows selected.
SCOTT@orcl >
SY.