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  1. #1
    Join Date
    Feb 2005
    Location
    Milwaukee, WI
    Posts
    105

    Unanswered: History/Data Change File Approach

    I need to record in a table:
    Who, When, What Field and New Value of Fields
    When changes occur to an existing record.

    The purpose is for users to occassionally view the changes. They'll want to be able to see the history of the record - who changed what and when.

    I figured I'd add the needed code to the stored procedure that's doing the update for the record.

    When the stored procedure is called to do the update, the PK and parameters are sent.

    The SP could first retain the current state of the record from the disk,
    then do the update, then "spin" thru the fields comparing the record state prior to the update and after. Differences could be parsed to a "Changes string" and in the end, this string is saved in a history record along with a few other fields:

    Name, DateTime, Changes

    FK to Changed Record: some int value
    Name: Joe Blow
    Date: 1/1/05 12:02pm
    Changes: Severity: 23 Project: Everest Assigned Lab: 204

    How does the above approach sound?

    Is there a better way you'd suggest?

    Any sample code for a system that spins thru the fields comparing 1 temporary record with another looking for changes?

    Thanks,

    Peter

  2. #2
    Join Date
    Feb 2005
    Posts
    78
    Have you considered using a trigger? You can use the inserted and deleted tables to compare your data without having to save it manually. Then insert your data into the history table as you suggested.

    We often just save the before image to the history table along with the type of operation performed, the id that was used to perform it and a time stamp. Inserted records are not recorded (because all their data is already recorded on the live table) but deleted ones are. The differences for updated records can be determined at any time by comparing the before image to the next or previous stored image or the current actual record. We rarely actually look at this sort of history however unless data disappears or the customer tells us that there is something else wrong with the data and we need to trace what happened to it.

  3. #3
    Join Date
    Feb 2005
    Location
    Milwaukee, WI
    Posts
    105
    ejustuss - thanks for the thoughts.

    Good idea about simply saving the before image prior to the actual save of the new one.

    Our users are used to systems where they can click a button and see essentially the change history of the record. This particular system is a Work Order system. The status of the WO changes over time, etc.

    In other non-SQL Server systems I've developed I have a routine that prior to actual save:

    1. saves "before save" copy of record
    2. updates the record with new values into the DB
    3. peels off an "After save" copy of record
    4. runs a routine that compares side by side each field. Any changes are noted in a text variable (field name, new value).
    5. Once all the fields are spun thru (compared), if there is any information in the text variable, a "change table" record is created with FK to the parent record, Who Changed it, When changed, and a single text field describing all the changes.

    Weeks later when a user is viewing the particular record, they can press a button and have a query run against the change table to bring up a simple list of what changed when.

    One wrinkle is that a Work Order has a huge text area. Once a WO is accepted by a lab, this text are becomes "frozen". So if we simply peel off a before save copy each time a user specifies an update - I wouldn't want to needlessly include this particular field due to space considerations.

    Bottom line - I was assuming someone might have a canned routine for spinning thru a record comparing all field values against an identical layed out record. I figured there might be a system function or 2 to:
    1. Identify how many fields are in a table
    2. identify the content of a field - something like @@Field(i)
    where i=1 to number of fields in the table.

    Peter

  4. #4
    Join Date
    Feb 2005
    Posts
    78
    I don't know of any function like that although I am sure you can write one.

    In triggers you can use IF UPDATE(column) to test if a column is updated but you still have to use the column names which means writing a different trigger for each case . The data in text fields will probably not be accessible within the trigger.

    You don't have to save all the fields if you do a before image either just the fields you want to compare in case they are updated.

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