Diff between connect by prior empno = mgr & connect by empno = prior mgr

Question

Hi ,
Can any body tell me why both query give different result please tell me why
select empno, mgr,ename, level
from scott.emp
start with ename = 'JONES'
connect by prior empno = mgr
select empno, mgr,ename, level
from scott.emp
start with ename = 'JONES'
connect by empno = prior mgr

Answer 1

Hi,
nav wrote:
Hi ,
Can any body tell me why both query give different result please tell me whyWhy would you expect them to be the same?
PRIOR empno = mgris different from
empno = PRIOR mgrin the same way that
SQRT (x) = ymeans something other than
x = SQRT (y)
select empno, mgr,ename, level
from scott.emp
start with ename = 'JONES'
connect by prior empno = mgr This is what's called a Top-Down Query , where you find 'JONES' and all its descendants (as you normaly look at the hierachy).
On each level (>1), a new row will join the result set if its mgr is the same as the empno of a row on the prior level.
For example: the START WITH clause says that 'JONES' (empid=7566) will be on LEVEL=1.
Who will be on LEVEL=2? Any row that satisfies the CONNECT BY condiition "PRIOR empno = mgr". Since 7566 is the only empno on LEVEL=1, that is equivalent to saying "7566 = mgr". There happen to be 2 rows that meet that condition: ename='SCOTT' (empno=7788) and ename='FORD' (empno=7902).
Who will be on LEVEL=2? Any row that satisfies the CONNECT BY condiition "PRIOR empno = mgr". Since the rows with empnos 7788 and 7902 are the only rows on LEVEL=1 in this case, the condition is equivalent to saying "7788 = mgr OR 7902 = mgr". There happen to be 2 rows that meet that condition: ename='ADAMS' (empno=7876) and ename='SMITH' (empno=7369).
Who will be on LEVEL=3? Any row that satisfies the CONNECT BY condiition "PRIOR empno = mgr", which, in this case, is equivalent to saying "7876 = mgr OR 7369 = mgr". There happen to be no rows whcih meet that condition.
select empno, mgr,ename, level
from scott.emp
start with ename = 'JONES'
connect by empno = prior mgrThis is called a Bottom-Up Query , where you find 'JONES' and all its ancestors (as you normaly look at the hierachy).
On each level, a new row will join the result set if its empno is the same as the mgr of a row on the prior level.
Edited by: Frank Kulash on Apr 28, 2011 1:38 PM