Oracle Connect By 似乎产生了太多行

Question

Oracle Database 12c 企业版 12.1.0.2.0 版

我想我只是遗漏了一些东西，但是如果我运行这个查询没有 "connect by"，我会得到 2 行。当我添加 "connect by level <= 4" 时，我希望得到这 2 行中的每一行 4 次。实际结果不同

任何人都可以帮助我了解这里发生的事情吗？我不是在寻找一种只将每一行重复 4 次的解决方案——我已经找到了。我只是想了解发生了什么以及为什么。

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

非常感谢数学家。他在下面的答案中提供的第二个 link 正是我要找的。具体来说：

  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.

我从那个例子中很清楚，但我很难用简洁的语言表达出来。

Answer 1

除了 "level <= 4" 之外没有其他条件，原始 table、视图等（在本例中来自连接）的每一行都将在级别 2 生成两行，然后再生成四行第 3 级的行，第 4 级的 8 行。"Connect by" 本质上是一连串的联接，如果您没有使用 PRIOR 运算符的条件，则您正在执行交叉联接。

您可能想要添加 "and prior a.id = a.id"。这将导致 Oracle 抱怨循环（因为当 Oracle 在受 PRIOR 约束的列中看到相同的值时，它决定达到循环）。这又可以通过添加第三个条件来解决，通常是 "and prior sys_guid() is not null".

（已编辑；原始答案参考了 NOCYCLE，使用 "prior sys_guid() is not null" 方法时不需要它。）

最近在 OTN 上对此进行了讨论：https://community.oracle.com/thread/3999985

这里讨论了同样的问题：https://community.oracle.com/thread/2526535

Answer 2

为了说明 Mathguy 的回答，您在 CONNECT BY 子句中遗漏了一些谓词：

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

另一种方法是使用递归 with 子句：

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

Oracle Connect By 似乎产生了太多行

Oracle Connect By seems to produce too many rows

oracle

connect-by

hierarchical-query