Query regarding connect by

Question

I have the following table named student:----
NAME
a
b
c
d
Now I came across a clause connect by but I can't understand how it works.I know it has something to do with tree structures.But when I issued the following query :--
select * from student connect by prior name>name;
-----the following report was produced:-------
NAME
a
b
a
c
a
b
a
d
a
b
a
NAME
c
a
b
a
15 rows selected.
I really don't understand how it works.Please help.
Thanks in advance
Edited by: user11930797 on Apr 27, 2010 5:02 AM
Edited by: user11930797 on Apr 27, 2010 5:03 AM
Edited by: user11930797 on Apr 27, 2010 5:04 AM

Answer 1

Maybe it helps if you select the level and the complete path, like this:
SQL> with student as
  2  ( select 'a' name from dual union all
  3    select 'b' from dual union all
  4    select 'c' from dual union all
  5    select 'd' from dual
  6  )
  7   select name
  8        , level
  9        , sys_connect_by_path(name,' --> ') scbp
10     from student
11  connect by prior name > name
12  /
N      LEVEL SCBP
a          1  --> a
b          1  --> b
a          2  --> b --> a
c          1  --> c
a          2  --> c --> a
b          2  --> c --> b
a          3  --> c --> b --> a
d          1  --> d
a          2  --> d --> a
b          2  --> d --> b
a          3  --> d --> b --> a
c          2  --> d --> c
a          3  --> d --> c --> a
b          3  --> d --> c --> b
a          4  --> d --> c --> b --> a
15 rows selected.Now it shows you are building up 4 trees. One for each record, because there is no "start with" clause specified. And then the tree is built up using the clause "prior name > name", resulting in that output.
Hope this helps.
Regards,
Rob.