1

Oracle Database 12c Enterprise Edition Release 12.1.0.2.0

I expect I'm just missing something, but if I run this query without the "connect by", I get 2 rows. When I add "connect by level <= 4", I would expect to get each of those 2 rows 4 times. The actual result is different.

Can anyone help me understand what's happening here? I'm not looking for a solution that only repeats each row 4 times - I've already got that. I'm just looking to understand what's happening and why.

with alpha as (
        select 1 as id
            from dual
    ),
    beta as (
        select 1 as alpha_id,
                1 as beta_no
            from dual
        union all
        select 1 as alpha_id,
                2 as beta_no
            from dual
    )
select a.id,
        b.beta_no,
        level as the_level
    from alpha a
        inner join beta b
            on b.alpha_id = a.id
    connect by level <= 4
    order by a.id,
        b.beta_no,
        level
;

    ID    BETA_NO  THE_LEVEL

     1          1          1
     1          1          2
     1          1          2
     1          1          3
     1          1          3
     1          1          3
     1          1          3
     1          1          4
     1          1          4
     1          1          4
     1          1          4
     1          1          4
     1          1          4
     1          1          4
     1          1          4
     1          2          1
     1          2          2
     1          2          2
     1          2          3
     1          2          3
     1          2          3
     1          2          3
     1          2          4
     1          2          4
     1          2          4
     1          2          4
     1          2          4
     1          2          4
     1          2          4
     1          2          4

30 rows selected

Many thanks to mathguy. The second link he provided in the answer below had exactly what I was looking for. Specifically:

  1  with t as (select 1 as id from dual union all
  2             select 2 from dual)
  3  --
  4  select id, level
  5        ,prior id
  6        ,sys_connect_by_path(id,'=>') as cpath
  7  from   t
  8* connect by level <= 3
SQL> /

        ID      LEVEL    PRIORID CPATH
---------- ---------- ---------- --------------------------------------------------
         1          1            =>1
         1          2          1 =>1=>1
         1          3          1 =>1=>1=>1
         2          3          1 =>1=>1=>2
         2          2          1 =>1=>2
         1          3          2 =>1=>2=>1
         2          3          2 =>1=>2=>2
         2          1            =>2
         1          2          2 =>2=>1
         1          3          1 =>2=>1=>1
         2          3          1 =>2=>1=>2
         2          2          2 =>2=>2
         1          3          2 =>2=>2=>1
         2          3          2 =>2=>2=>2

14 rows selected.

It's clear to me from that example, but I'd be hard-pressed to succinctly put it into words.

4

2 回答 2

2

With no condition other than "level <= 4", every row from the original table, view etc. (from the join, in this case) will produce two rows at level 2, then four more rows at level 3, and 8 more at level 4. "Connect by" is essentially a succession of joins, and you are doing cross joins if you have no condition with the PRIOR operator.

You probably want to add "and prior a.id = a.id". This will lead to Oracle complaining about cycles (because Oracle decides a cycle is reached when it sees the same values in the columns subject to PRIOR). That, in turn, is solved by adding a third condition, usually "and prior sys_guid() is not null".

(Edited; the original answer made reference to NOCYCLE, which is not needed when using the "prior sys_guid() is not null" approach.)

This has been discussed recently on OTN: https://community.oracle.com/thread/3999985

Same question discussed here: https://community.oracle.com/thread/2526535

于 2017-01-11T01:01:38.853 回答
0

To illustrate Mathguy's answer, you are missing some predicates out of your CONNECT BY clause:

with alpha as (
        select 1 as id
            from dual
    ),
    beta as (
        select 1 as alpha_id,
                1 as beta_no
            from dual
        union all
        select 1 as alpha_id,
                2 as beta_no
            from dual
    )
select a.id,
       b.beta_no,
       level as the_level
from   alpha a
       inner join beta b
         on b.alpha_id = a.id
connect by level <= 4
           AND PRIOR a.id = a.id
           AND PRIOR b.beta_no = b.beta_no
           AND PRIOR sys_guid() IS NOT NULL
order by a.id,
         b.beta_no,
         LEVEL;

        ID    BETA_NO  THE_LEVEL
---------- ---------- ----------
         1          1          1
         1          1          2
         1          1          3
         1          1          4
         1          2          1
         1          2          2
         1          2          3
         1          2          4

An alternative would be to use the recursive with clause:

with alpha as (
        select 1 as id
            from dual
    ),
    beta as (
        select 1 as alpha_id,
                1 as beta_no
            from dual
        union all
        select 1 as alpha_id,
                2 as beta_no
            from dual
    ),
    multiply (id, beta_no, rn) AS (SELECT a.id,
                                          b.beta_no,
                                          1 rn
                                   FROM   alpha a
                                          INNER JOIN beta b
                                            ON a.id = b.alpha_id
                                   UNION ALL
                                   SELECT ID,
                                          beta_no,
                                          rn + 1
                                   FROM   multiply
                                   WHERE  rn + 1 <= 4)
SELECT ID,
       beta_no,
       rn AS the_level
FROM   multiply
order by id,
         beta_no,
         rn;

        ID    BETA_NO  THE_LEVEL
---------- ---------- ----------
         1          1          1
         1          1          2
         1          1          3
         1          1          4
         1          2          1
         1          2          2
         1          2          3
         1          2          4
于 2017-01-11T11:35:23.137 回答