使用先验连接显示数据的父级？

Question

我有这个 table 我有

parentitem    |    childitem     ---table name
  dinner      |      steak
  dinner      |      wine
  dinner      |      mashed potato
  dinner      |      coffee
  coffee      |      sugar
  coffee      |      water
  dinner      |      cake
  cake        |      liquid syrup
  cake        |      egg

我想使用 connect 检索 'dinner' 的所有子项，之前我使用了 ff 代码

 Select  Level, LPAD('->',2*(LEVEL-1))||CHILDITEM     From table  
 Start With parentitem = 'dinner'    Connect By Prior childitem =
 parentitem

但它不包括 'dinner' 的父项，但它可以正确生成晚餐的所有子项，顺便说一句，我的朋友提示我使用 union。我正在使用甲骨文。

所以我的预期结果是

LEVEL  |   CHILDITEM
  0    |  dinner
  1    |  steak
  1    |  wine
  1    |  mashed potato
  1    |  coffee
  2    |  sugar
  2    |  water
  1    |  cake
  2    |  liquid syrup
  2    |  egg

Answer 1

问题是您的数据中没有 "dinner" 作为子项。

如果你这样做了，这将是一件简单的事情 START WITH childitem='dinner'。

例如，

SELECT LEVEL,
       LPAD ('->', 2 * (LEVEL - 1)) || childitem
FROM   t
START WITH childitem = 'dinner'
CONNECT BY PRIOR childitem = parentitem

由于您的数据，如果您希望 "dinner" 列为子项，您需要在查询中创建一个不存在的行。 UNION ALL 是一个很好的方法。例如，

SELECT 0 AS "LEVEL",
       'dinner' childitem
FROM   DUAL
UNION ALL
SELECT LEVEL,
       LPAD ('->', 2 * (LEVEL - 1)) || childitem
FROM   t
START WITH parentitem = 'dinner'
CONNECT BY PRIOR childitem = parentitem

另一种方法是使用 UNION ALL 创建源数据中缺失的行。例如，

SELECT LEVEL,
       LPAD ('->', 2 * (LEVEL - 1)) || childitem
FROM   (SELECT NULL parentitem,
               'dinner' childitem
        FROM   DUAL
        UNION ALL
        SELECT parentitem,
               childitem
        FROM   t) t
START WITH childitem = 'dinner'
CONNECT BY PRIOR childitem = parentitem