02 February 2010

Reports - Ways of Looking at Data – Part II

    In a previous post on the same topic I’ve started to exemplify how the logic for reports could be built using the PO Header/Details example provided by AdventureWorks. The queries were looking mainly at Vendor and PO Header data, the next logical step being the introduction of PO Details and Products. If the header contains general information about a PO that applies to all PO Lines, the PO Lines contain the reference to the purchased Product, Quantities (e.g. Ordered, Receipt, Rejected, etc.) & UOM, Prices (e.g. Unit Price), Reference Dates (e.g. Due Date), each PO Header having one or more PO Lines.

    Following PO Header’s example and logical partitioning , could be created a view based only on PO Details and the child tables, though as in this case there are no child tables – does it makes sense to create a view? Yes, why not?! The logic encapsulated by a view doesn’t resume only to joining tables, but also to provide a restrained list of attributes, computed columns or limit the scope by using constraints.
    As can be seen from the above query, has been introduced several computed columns: OpenQty – the Quantity remaining to be receipt calculated as the difference between OrderedQty and ReceiptQty, ReceiptTotal – the total value of quantities receipt, OutstandingTotal – the total value corresponding to OpenQty. These three columns are quite often met in analysis and even if their definition is quite easy to follow and implement, it makes sense to include them in the view, thus reducing logic replication. In query writing best practices I was recommending to encapsulate formulas and business logic in functions, however in this case the logic is quite simple and it doesn’t makes sense to create a function for each formula used in a query, the recommendation should be used in theory (complex) formulas that are often reused.

    The alternative for partitioned view is to create an extended view that include PO Header and PO Details, such type of view is really easy to use by most of the users. I prefer to use both approaches, having thus more flexibility.
    Do not forget to check the variations in cardinality! This exercise allow also to identify whether the referential integrity is kept. Even if it’s hard to believe, I found referential integrity broken in databases designed to enforce it, therefore it’s a good idea to check whether the referntial integrity is kept between the various elements.
    The same query could be also created also by using logical partitioned view, though before doing that we have to create a view also for Products:
    The final query based on logical partitioned views:
    Before using the data there are 1-2 tests that needs to be done in order to assure ourselves that the data available at the various levels of details are synchronized: is the case of the SubTotal which should match the cumulated LineTotal for each line.
    Simple reports based on PO Detail level resume in retrieving the POs matching a certain criteriea: closed POs, Open POs, POs with overreceipt, with rejections or any other business case.
    The * (ALL) has been used only in order to minimize query’s length, as a good practice avoid using the “*” and provide instead only the list of attributes needed!

    Other types of reports are showing the data at different aggregations, for example calculating the Total Amount spent for each Vendor/Product, the Outstanding Amount, etc.
    More complex reports can be built with the help of window functions, for example showing the last PO Unit Price per Vendor/Product, such a report being quite useful when reevaluating the Standard Costs.

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