my $SQL_query;
   
   $SQL_query = qq {SELECT A.DISPLAY_ORDER, A.NAME 
PARENT_NAME, B.NAME CHILD_NAME
FROM CATEGORY A, CATEGORY B 
WHERE B.ID_PARENT_CATEGORY(+) = A.ID_CATEGORY 
AND A.DISPLAY_ORDER != -1 ORDER BY DISPLAY_ORDER};
[download]

Just a suggestion -- Oracle, SQL Server, Sybase, and others tend to use idiosyncratic ways of expressing joins. Using the ANSI join specifications will make your code more portable between RDBMS implementations. For example:

SELECT A.DISPLAY_ORDER, A.NAME PARENT_NAME,  B.NAME CHILD_NAME 
FROM CATEGORY A
LEFT OUTER JOIN  CATEGORY B 
ON B.ID_PARENT_CATEGORY = A.ID_CATEGORY
[download]

I know that Sybase, SQL Server, Postgres and Oracle all support ANSI joins, and I suspect there are others as well.

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"Computeri non cogitant, ergo non sunt"