Connect By Without Prior

Question

Hi,
In 2005, this link (AskTom) discussed connect by without prior and Tom Kyte said:
"I shall file a documentation bug to have the documentation updated to accurately reflect reality."In 2007 11g was released and Oracle 11g doc states:
* In a hierarchical query, one expression in condition must be qualified with the PRIOR operator to refer to the parent row.
PRIOR is most commonly used when comparing column values with the equality operator. (The PRIOR keyword can be on either side of the operator.) PRIOR causes Oracle to use the value of the parent row in the column. Operators other than the equal sign (=) are theoretically possible in CONNECT BY clauses. However, the conditions created by these other operators can result in an infinite loop through the possible combinations. In this case Oracle detects the loop at run time and returns an error.But connect by without prior is still allowed in 11g
Oracle Database 11g Enterprise Edition Release 11.1.0.6.0 - Production
PL/SQL Release 11.1.0.6.0 - Production
CORE 11.1.0.6.0 Production
TNS for Linux: Version 11.1.0.6.0 - Production
NLSRTL Version 11.1.0.6.0 - Production
select level lvl from dual connect by level <= 10
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10Does this mean that connect by without prior is now officially allowed?

Answer 1

Bishòp wrote:
In 2007 11g was released and Oracle 11g doc states:
* In a hierarchical query, one expression in condition must be qualified with the PRIOR operator to refer to the parent row.
PRIOR is most commonly used when comparing column values with the equality operator. (The PRIOR keyword can be on either side of the operator.) PRIOR causes Oracle to use the value of the parent row in the column. Operators other than the equal sign (=) are theoretically possible in CONNECT BY clauses. However, the conditions created by these other operators can result in an infinite loop through the possible combinations. In this case Oracle detects the loop at run time and returns an error.
Tom saw it as a bug in the documentation because of the way that Oracle allows connect by without prior to be used.
I know some people who still argue that the documentation says you shouldn't do a connect by without a prior and that it will be unsupported, but...
a) In 10.1.0.2 if you did "select rownum from dual connect by rownum <= 10" it would actually return 11 rows and this was "corrected" in 10.2 so that it returned 10 rows. Why would Oracle correct something that they didn't support?
b) I think the key thing is to try and understand what is being said by the documentation...
In a hierarchical query_, one expression in condition must be qualified with the PRIOR operator to refer to the parent row.
A hierarchical query is where you want to refer to a parent row, but are we doing that with our "select rownum|level from dual connect by rownum|level <= X" statements that don't have a PRIOR in them? I would consider it that we are not asking for a "hierarchical" query but are in fact just asking for it to connect to itself, therefore creating a loop, while the loop condition holds true.
In fact, if you try and use PRIOR with rownum or level you get...
SQL> ed
Wrote file afiedt.buf
  1  select rownum
  2  from dual
  3* connect by rownum = prior rownum + 1
SQL> /
connect by rownum = prior rownum + 1
ERROR at line 3:
ORA-00976: LEVEL, PRIOR, or ROWNUM not allowed here
SQL>which is actually telling you, in no uncertain terms, that if you are going to use PRIOR then you can't use level or rownum, and conversely, if you are going to use level or rownum then you can't use PRIOR.
This would therefore indicate that it is expected behaviour and it is just the documentation that is worded badly and often taken wrongly.
;)