09 February 2010

Hierarchies – The Oracle Solution

    In yesterday’s post on hierarchy-based reports I was showing, using a BOM example, how hierarchy-related problems could be approached using common table expressions introduced starting with SQL Server 2005. Oracle’s approach to hierarchical queries is based on CONNECT BY clause that allows to specify the join constraint between the table(s) used in the recursive logic and previous output. If in SQL Server the logic is split in two parts – the anchor member sub-query representing the starting point of the query, respectively the recursive member, the query on which the recursive technique is applied, in Oracle the two are combined in only one query with the help of START WITH clause. Here’s the basic structure for building a hierarchy based on BillOfMaterials table coming with AdventureWorks.
    The START WITH ASS.productassemblyid IS NULL and TRUNC(NVL(ass.enddate, SYSDATE))>=TRUNC(SYSDATE) constraints function as initialization for the recursive logic, being ignore in next iterations, while the CONNECT BY PRIOR ass.componentid = ass.productassemblyid and TRUNC(NVL(ass.enddate, SYSDATE))>=TRUNC(SYSDATE) constraints function as join constraints between the prior result set and current logic. Maybe somebody is asking why the ‘Active BOM’ constraint if specified in both START WITH, respectively CONNECT BY clauses and not in the WHERE clause? If we are interested only in the Active BOMs then the corresponding constraint must be added to the initiating clause, and is added to the CONNECT BY clause because we need to assure that we take only the active children. If we move the ‘Active BOM’ constrain from the CONNECT BY to WHERE clause then most probably we’ll get a totally different number of records, respectively 9525 vs. 8730 how much the above query returns. The easiest way to check whether the logic is correct is to do include the Activer ecords in an inline view and construct with it the BOM.
    As the above query returns 8730 records it seems that the above logic is correct.

    In order to match the ‘CTE-Based BOM structure’ query from the previous post, the Oracle query needs several further changes, namely to construct the Path, add the top-most ComponentID, calculate the TotalQty, and add the Product details. The Path can be constructed using the SYS_CONNECT_BY_PATH function using as parameters the attribute needed to be concatenated (e.g. componentid) and the delimiter (e.g. ‘\’). The top-most ComponentID can be retrieved using the CONNECT_BY_ROOT operator. Unfortunately Oracle doesn’t provides a straight way to calculate the TotalQty, though there is an alternative, using again the SYS_CONNECT_BY_PATH function with the PerAssemblyQty attribute and multiplication sign ‘*’ as parameters, this smart trick allowing to evaluate the final expression (e.g. ‘1*1*2*5’ will be evaluated to 10); because the extract is typically saved to excel, the final concatenation could be written as an Excel formula expression (e.g. ‘=1*1*2*5’). Here is the final query:
    The specification of an ORDER BY is not always required because typically the output of a hierarchical query in Oracle is sorted as if it were used the depth-first search, though if I remember correctly there are exceptions too.

    Sometimes it’s useful to create a concatenation of Products in order to see the actual structure of the BOM (e.g. ‘\BK-R93R-62\BB-9108\BE-2349\BA-8327’) or to use to use incremental spaces instead (see. IndentPath), for this needing to bring the Product join for ComponentID into the recursive logic.
    By comparing the SQL Server vs. Oracle implementation for hierarchical queries, I would say that Oracle’s implementation is much simpler to use from the creational point of view with a trade off in functionality, several functions being required in order to extract information from top-most record or to concatenate attributes. On the other side SQL Server provides a more elegant solution that allows more functional flexibility in the detriment of code duplication – the anchor vs. the recursive member.

    In case you need to load the Product and BillOfMaterials tables in Oracle, you could do this by using the ‘SQL Server Import and Export Wizard’, for the steps see the SQL Server to Oracle Data Export – Second Magic Class tutorial.

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