Weblog for the AMIS Technology corner
Posts tagged hierarchical query
Oracle RDBMS 11gR2 – new style hierarchical querying using Recursive Subquery Factoring
Sep 29th
Oracle Database 11g Release 2 introduces the successor to the good old Connect By based hierarchical querying, called Recursive Subquery Factoring. The basics are described in a previous article: http://technology.amis.nl/blog/6104/oracle-rdbms-11gr2-goodbye-connect-by-or-the-end-of-hierarchical-querying-as-we-know-it. This article will show some additional examples of using this recursive subquery factoring syntax.
The essence of this recursiveness: the subquery consists of two queries unioned together. The first query returns the root-nodes, the starting points in the tree or network. The second query is used to continually retrieve the next step or level: it refers to the subquery itself and finds the next node level based on the levels (root and zero or more previously retrieved levels) already retrieved by the subquery.
In code:
Oracle RDBMS 11gR2 – goodbye Connect By or: the end of hierarchical querying as we know it
Sep 1st
Many years ago, Oracle basically set the standard in SQL. Whatever was Oracle SQL could be seen as the standard. This has never been absolutely true – ANSI SQL was different from Oracle SQL. In some respects the differences can be traced back to lack of functional richness in the standard. However, in certain areas, Oracle has walked its own path with certain functions and features with the rest of the RDBMS pack following another route. It seems to be as if starting with Oracle RDBMS 9iR2, Oracle has made several important steps towards embracing the ANSI SQL standard syntax, usually while maintaining its own specific flavor of those same functions.
Some examples of Oracle specific syntax that are also available through their ANSI SQL counterpart – that in most instances has even more functionality – and that is almost always the preferred approach going forward:
- outer join syntax: in addition to Oracle’s (+) notation, 9iR2 introduced the left outer join, right outer join and full outer join (like (+) on both ends of the join condition, something that Oracle does not support)
- the Decode function that with 9iR2 can (and should) be replace by the CASE expression
- the to_char function that in many instances can be replace by extract {year|month|day|HOUR | MINUTE | SECOND} from date-value
- the ltrim and rtrim functions that can be replaced by the trim function with leading and trailing settings
The 11gR2 release has another example of Oracle specific SQL that is now complemented with the ANSI SQL standard approach:the CONNECT BY syntax for hierarchical querying that can now be replaced by the ANSI SQL standard Recursive Subquery Factoring clause, which is an extension to the WITH clause introduced as long ago as Oracle7 and still relatively underused by Oracle SQL developers.
