Cameron Albert

Ramblings of software development, games and technology.

SQL 2005 XML Data Type, Stored Procedures and Lists

I've seen a lot of stuff out there regarding the SQL 2005 XML data type but most of it is just regurgitates the MSDN documentation. That's fine and all but what about practical uses of it? Well, I have a practical use sample. In building my persistent browser based game Perenthia I have a concept of a Place. A place is a virtual space in which objects are stored. For Perenthia the places represent the various rooms or tiles players move around on. The place or room has exits defined that allow the player to move from one place to the next. The exits are the typical directions; north, south, up, down, etc. In the database I have a Places table and a PlaceExits table. The Places table stores all the information regarding a place and the PlaceExits table stores the placeId along with a directionId and destinationId so I know what exits are available in any room and what rooms they lead to.

The simplified schema for the places would be:

 Places Tables

 In the stored procedure that retrieves the place information I use the following query snippet in the select clause:

    SELECT

        p.*,

    (
            SELECT
                e.DirectionId        AS "@directionId",
                e.DestinationId        AS "@destinationId"
            FROM
                dbo.PlaceExits e
            WHERE
                e.PlaceId = p.PlaceId
            FOR XML PATH('exit'), ROOT('exits')
        ) AS ExitsXml

    FROM dbo.Places p 

 This creates an XML fragment I can then parse in the application to fill a collection of Exits on the Place object.

When saving place information I pass XML generated from the Exits collection in a stored procedure like so:

CREATE PROCEDURE dbo.Places_SavePlace  (@PlaceId int, @ExitsXml xml)

From within the save procedure I perform an update or insert of the place data and then execute the following sql to insert and update the exits for the current place:

    — Exits

    — Process the existing exits first

    UPDATE
        dbo.PlaceExits
    SET
        DestinationId    = e.ex.value('(@destinationId)[1]', 'int')
    FROM
        @ExitsXml.nodes('/exits/exit') as e(ex)
    WHERE
        PlaceId = @PlaceId
        AND DirectionId = e.ex.value('(@directionId)[1]', 'tinyint')

    — Process any new exits

    INSERT INTO dbo.PlaceExits
    (
        ObjectId, DirectionId, DestinationId
    )
    SELECT
        @PlaceId,
        e.ex.value('(@directionId)[1]', 'tinyint'),
        e.ex.value('(@destinationId)[1]', 'int')
    FROM
        @ExitsXml.nodes('/exits/exit') as e(ex)
    WHERE
        e.ex.value('(@directionId)[1]', 'tinyint') NOT IN
        (
            SELECT DirectionId FROM dbo.PlaceExits WHERE PlaceId = @PlaceId
        )

This is working pretty well and keeps me from having to loop through the exits in the application and make multiple database calls. 

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Written by Cameron

November 14, 2007 at 5:41 pm

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