Writing query using UNION Operator

Question

Question -
(1)
Write an SQL Statement to list the following items: Customer ID, Customer Name, number of invoices, sum of total for invoices. Ensure that all customers are returned in the result set.
Answer for the above is written as below by one person. That seams to be correct. Is there another way of writing this same query.;
select c.CUSTOMER_ID,c.NAME,i.cnt,i.s
from gee_customer c,(select customer_id,count(*) as cnt, sum(TOTAL) as s
from gee_invoice
group by customer_id) i
where c.CUSTOMER_ID = i.customer_id (+)
(2)
My other question is How to write the above answer (or what you sugest) using UNION operator ?
Any ideas please
Message was edited by:
user483578

Answer 1

In fact the outer join means you use the union of two result sets - usual join result
and the rows from the outer table for which there is not any row in the inner table.
SQL> select d.deptno, e.ename from emp e, dept d
  2  where d.deptno = e.deptno(+)
  3  order by 1,2
  4  /
    DEPTNO ENAME
        10 CLARK
        10 KING
        10 MILLER
        20 ADAMS
        20 FORD
        20 JONES
        20 SCOTT
        20 SMITH
        30 ALLEN
        30 BLAKE
        30 JAMES
        30 MARTIN
        30 TURNER
        30 WARD
        40
15 rows selected.
SQL> select d.deptno,e.ename from emp e, dept d
  2  where d.deptno = e.deptno
  3  union
  4  select deptno, null
  5  from dept d where not exists (select null from emp e where e.deptno = d.deptno)
  6  order by 1,2
  7  /
    DEPTNO ENAME
        10 CLARK
        10 KING
        10 MILLER
        20 ADAMS
        20 FORD
        20 JONES
        20 SCOTT
        20 SMITH
        30 ALLEN
        30 BLAKE
        30 JAMES
        30 MARTIN
        30 TURNER
        30 WARD
        40
15 rows selected.In your example something like (NOT tested !)
with i as (select customer_id,count(*) as cnt, sum(TOTAL) as s
from gee_invoice group by customer_id)
select c.CUSTOMER_ID,c.NAME,i.cnt,i.s
from gee_customer c, i
where c.CUSTOMER_ID = i.customer_id
union
select c.CUSTOMER_ID,c.NAME,null,null
from gee_customer c
where not exists (select null from i where c.CUSTOMER_ID = i.customer_id)
Rgds.