Connect by prior (hierarchical query)

Question

Hi All,
Some1 asked me a question which goes like this:
Source
emp_no dep_no
110 10
110 20
110 30
110 40
120 10
130 10
130 20
write a query to achieve the following output from the above source
emp_no dept_no
110 10203040
120 10
130 1020
Now I have come across solutions with 'connect by prior' clauses but I am nt able to understand how oracle is producing that result , Could someone point me to a good article or text that can explain this concept very thoroughly( I have searched on google and got to see many articles but I couldnt able to understand it since these articles were not explaining everything)
Regards
Rahul

Answer 1

You can try this:
SQL> ed
Wrote file afiedt.buf
  1  SELECT deptno,MAX(TRIM(SUBSTR(SYS_CONNECT_BY_PATH(empid,','),2)))
  2         KEEP(DENSE_RANK LAST ORDER BY deptno) FROM
  3  (SELECT deptno,empid,ROW_NUMBER() OVER(PARTITION BY deptno ORDER BY deptno) curr,
  4                       ROW_NUMBER() OVER(PARTITION BY deptno ORDER BY deptno) - 1 prev
  5  FROM emp)
  6  CONNECT BY PRIOR curr = prev
  7  AND deptno = PRIOR deptno
  8  START WITH curr = 1
  9* group by deptno
SQL> /
    DEPTNO MAX(TRIM(SUBSTR(SYS_CONNECT_BY_PATH(EMPID,','),2)))KEEP(DENSE_RANKLASTORDERBYDEPTNO)
        10 1,2,3,4
        20 5,6,7
SQL> select deptno,empid from emp;
    DEPTNO      EMPID
        10          1
        10          2
        10          3
        10          4
        20          5
        20          6
        20          7
7 rows selected.
SQL>