在 Oracle 分层查询中放置 PRIOR 语句导致的差异

Question

使用 Connect By 时，您似乎可以在 Oracle 中执行以下两项操作。

 CONNECT BY NOCYCLE 
    parent_id = PRIOR child_r_object_id

 CONNECT BY NOCYCLE PRIOR 
    parent_id = child_r_object_id

有什么区别，网上大部分例子都倾向于使用第二种语法，但是两者都执行。

Answer 1

"prior"关键字父记录的字段。

例如这个

parent_id = PRIOR child_r_object_id

表示"child_r_object_id from parent record is equal to parent_id of child line"

PRIOR parent_id = child_r_object_id

意思正好相反——父记录的parent_id等于子记录的child_r_object_id

Answer 2

基本上您定义从上到下或从下到上构建层次结构。

查看此示例以了解不同之处：

WITH t(person, parent_id, ID) AS (
    SELECT 'Grandma', NULL, 1 FROM dual
    UNION ALL SELECT 'Mother', 1, 10 FROM dual
    UNION ALL SELECT 'Daughter', 10, 100 FROM dual
    UNION ALL SELECT 'Son', 10, 101 FROM dual)
SELECT person AS leaf_person, CONNECT_BY_ROOT(person) AS top_person, 
       SYS_CONNECT_BY_PATH(person, '->'), 'down' AS direction
FROM t
WHERE CONNECT_BY_ISLEAF = 1
START WITH parent_id IS NULL
CONNECT BY parent_id = PRIOR ID
UNION ALL
SELECT person as leaf_person, CONNECT_BY_ROOT(person) as top_person, 
       SYS_CONNECT_BY_PATH(person, '->'), 'up' AS direction
FROM t
WHERE CONNECT_BY_ISLEAF = 1
START WITH ID IN (100,101)
CONNECT BY PRIOR parent_id = ID;


+-----------------------------------------------------------------+
|LEAF_PERSON|TOP_PERSON|SYS_CONNECT_BY_PATH(PERSON,'->')|DIRECTION|
+-----------------------------------------------------------------+
|Daughter   |Grandma   |->Grandma->Mother->Daughter     |down     |
|Son        |Grandma   |->Grandma->Mother->Son          |down     |
|Grandma    |Daughter  |->Daughter->Mother->Grandma     |up       |
|Grandma    |Son       |->Son->Mother->Grandma          |up       |
+-----------------------------------------------------------------+

通常您只有一个根（即 parent_id IS NULL）或至少确定根元素，因此大多数示例使用 "top-to-down" 方向。

在 Oracle 分层查询中放置 PRIOR 语句导致的差异

Difference caused by placement of PRIOR statement in an Oracle hierarchical query

sql

oracle

hierarchical