CONNECT BY loop in user data - how to stop on loop?

Question

Hey,
I have a table EXAMPL_TAB with two columns PART_1 and PART_2.
looks like:
PART_1  |  PART_2
    z1  |      z2
    z2  |      z3
    z3  |      z4
    z3  |      z2    <-- LOOP  -- don't wanna loop again
    z3  |      z1    <-- LOOP  -- don't wanna loop again
    z4  |      z5I wolud get sth with SYS_CONNECT_BY_PATH(PART_1, '/') like:
/z1
/z1/z2/
/z1/z2/z3/
/z1/z2/z3/z4
/z1/z2/z3/z4/z5or better
/z1
/z1/z2/
/z1/z2/z3/
/z1/z2/z3/z2  <-- and stop
/z1/z2/z3/z1  <-- and stop
/z1/z2/z3/z4
/z1/z2/z3/z4/z5Please, somebody help me.

Answer 1

like
SQL> with t as (
  2  select 'z1' c1,'z2' c2 from dual union all
  3  select 'z2' c1,'z3' c2 from dual union all
  4  select 'z3' c1,'z4' c2 from dual union all
  5  select 'z3' c1,'z2' c2 from dual union all
  6  select 'z3' c1,'z1' c2 from dual union all
  7  select 'z4' c1,'z5' c2 from dual  )
  8  select c1,c2,level
  9  from t
10  start with c1 = 'z1'
11  connect by nocycle prior c2 = c1;
C1 C2      LEVEL
z1 z2          1
z2 z3          2
z3 z4          3
z4 z5          4
z3 z1          3