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Recursive Query

On our Oracle 9x database, I have:
SQL> select * from R_Rightsassociation;

Right.5  Right.3
Right.3  Right.4
Right.4  Right.2
The result set I want is:
 
Right.5  Right.3
Right.5  Right.4
Right.5  Right.2
Right.3  Right.4
Right.3  Right.2
Right.4  Right.2
(I'm doing this because the desired result set will be used for super fast, simple select queries through a simple where clause).
What oracle SQL query would you suggest to get the desired result set?

This problem looks very similar to one that appeared on the Oracle Metalink SQL*Plus forum on December 10, titled "Recursive Query". Michael Shoblock answered that question, recommending CONNECT BY and SYS_CONNECT_BY_PATH. Here's what he meant:
SELECT    SUBSTR ( path
                 , 1
                 , INSTR ( path
                         , ' / ' -- This separator must 
                                 --     not occur in names
                         ) - 1
                 )  AS ancestor
,         child
FROM      (  -- Begin in-line view to derive path
          SELECT      parent
          ,           child
          ,           SUBSTR ( SYS_CONNECT_BY_PATH ( parent
                                                   , ' / '
                                                   )
                             , 1 + LENGTH (' / ')
                             )  -- Don't want the separator 
                                --     at the beginning
                      || ' / '  AS path
                      -- Separator at end ensures that INSTR
                      --      (above )never returns 0
          FROM        r_rightsassociation
          CONNECT BY  parent = PRIOR child
          )  -- End  in-line view to derive path
ORDER BY  ancestor
,         child
;
I assume that the table r_rightsassociation contains two columns, parent and child, and that the ideal result set shows all ancestor/descendant relationships. Oracle has always supported hierachical queries, based on this kind of tree structure, but until version 9 the ancestor/descendant relationship was only expressed by the LEVEL pseudo-column and the relative position of rows in the result set. In version 9, Oracle introduced the SYS_CONNECT_BY_PATH function, which makes it much easier to show ancestor/descendant relationships on each row, which is exactly what you want.

SYS_CONNECT_BY_PATH returns a string, consisting of a given column from each level of the hierarchy, starting with the root (where LEVEL = 1) and ending with the current row. Each of these column values is preceded by a separator string. Each row's ultimate ancestor is represented by the first item in this string. The solution above assumes that the string ' / ' never occurs as part of a parent; if it does, pick some other string. It doesn't really matter how long or ugly, since you never display it.

This query has no START WITH clause, so every row in the table will be treated as a root.

In your sample data, each node has (at most) one parent and (at most) one child. The solution works even if a parent has several children (e.g., if you add a row Right.3 Right.1) or if a child has several parents (Right.6 Right.3). If both of these conditions are true (Right.5 Right.4) the solution still works, but the result set includes n identical rows when there are n separate paths by which an ancestor is related to a descendant. If that's a problem, simply change the main query to begin with SELECT DISTINCT.

This was last published in January 2005

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