Reduction columns are not working for group by expression in oracle

Question

I tried to concat two columns and using group by expression, but it is not work. how can I grouped multiple columns in oracle. first_name and last_name are reduction data.

SELECT employee_id,
  employee_name,
  employee_unique
FROM
  (SELECT a.id AS employee_id,
    (a.first_name
    || a.last_name) AS employee_name,
    b.employee_unique
  FROM A a
  INNER JOIN b
  ON a.id=b.employee_id
  GROUP BY a.id,
    b.employee_unique,
    (a.first_name
    || a.last_name)
  );

Answer 1

What does "not work" mean? It works for me (though, as I don't have your tables, I used Scott's EMP and DEPT, but everything else is more or less the same). If it isn't correct, you should explain what is wrong with it. If you want us to work with your data, please, provide CREATE TABLE and INSERT INTO sample data.

SQL> select employee_id,
  2         employee_name,
  3         employee_unique
  4  from
  5    (select a.empno          as employee_id,
  6            a.ename || a.job as employee_name,
  7            b.dname          as employee_unique
  8    from emp a
  9    inner join dept b
 10       on a.deptno=b.deptno
 11    group by a.empno,
 12             b.dname,
 13             a.ename || a.job
 14    );

EMPLOYEE_ID EMPLOYEE_NAME       EMPLOYEE_UNIQU
----------- ------------------- --------------
       7654 MARTINSALESMAN      SALES
       7876 ADAMSCLERK          RESEARCH
       7566 JONESMANAGER        RESEARCH
       7698 BLAKEMANAGER        SALES
       7844 TURNERSALESMAN      SALES
       7369 SMITHCLERK          RESEARCH
       7788 SCOTTANALYST        RESEARCH
       7900 JAMESCLERK          SALES
       7902 FORDANALYST         RESEARCH
       7782 CLARKMANAGER        ACCOUNTING
       7934 MILLERCLERK         ACCOUNTING
       7499 ALLENSALESMAN       SALES
       7521 WARDSALESMAN        SALES
       7839 KINGPRESIDENT       ACCOUNTING

14 rows selected.

SQL>

Though, as there's nothing really to be grouped (no aggregation here), you could have used distinct (without group by clause) and get the same result:

select employee_id,
       employee_name,
       employee_unique
from
  (select distinct
          a.empno          as employee_id,
          a.ename || a.job as employee_name,
          b.dname          as employee_unique
  from emp a
  inner join dept b
     on a.deptno=b.deptno
  );