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How to count the number of specific values in a field, then display the result in another field.

  1. #1
    JJAMII is offline Novice
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    How to count the number of specific values in a field, then display the result in another field.

    Hi

    I am creating a database for a games rental system. I have one table which is titled 'Games' and another which is titled 'Game Copies'. To add a Game Copy, you must first add the Game in to the Games table. This will mean entering a title etc. When you then go to add a game copy, there is a combo box which will drop down and show all the current titles from the Games table so that you can select the game you want and add a copy of it to the Copies table.

    In the Games table, there is an attribute for 'Copies Held'. Is there some way that i can have the database count how many copies of the Game are in the 'Game Copies' table, and display it in this field?

    Thanks, Jamie.

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    nick404 is offline Competent Performer
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    You can create a field (let's say totalCopies) in your Game Copies table that sums the # of copies, then create an identical field in your Games table and set a 1:1 relationship between the two fields. I have been away from access for the last 4 months due to school so there may be a better way but this is my initial solution.

  3. #3
    JJAMII is offline Novice
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    The problem there is that there is already a One to Many relationship from Game, to Game Copies.

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    nick404 is offline Competent Performer
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    I am having a hard time visualizing your table(s) setup. Can you post a screenshot of the relationships screen? I am wondering if it would be better to have your Game Copies table consist of a PK of the game title then a field numCopies that can be incremented when another copy is added to the system.

  5. #5
    JJAMII is offline Novice
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    I am actually having a pretty hard time with the database as a whole. It's for an assignment at university. We were given sets of data to normalize and drew up ERDs based on the normalization process. My ERD is almost identical to the one the lecturer uploaded as an example of how it should be, so that is apparently correct. But now that i'm trying to implement the planning into a database the structure just doesn't seem to make sense.

    But yeah anyway, sorry to ramble, here are the relationships and my final ERD.




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  6. #6
    June7's Avatar
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    Why do you have CurrentRental and RentalHistory tables? Should be 1 table - Rentals.

    I would not advise 'incrementing' a field. Saving aggregate data is usually a bad design. Enter raw data and calculate summary data when needed. CopiesHeld should be a calculated, not stored, value.

    Title should not be in both Games and GameCopy, only in Games.

    Advise no spaces or special characters/punctuation (underscore is exception) in naming convention.
    How to attach file: http://www.accessforums.net/showthread.php?t=70301 To provide db: copy, remove confidential data, run compact & repair, zip w/Windows Compression.

  7. #7
    JJAMII is offline Novice
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    Yeah i wasn't sure about Rental History / Current Rental. I just added the Current Rental table not long ago as i was struggling to see how i would be able to issue rentals without it. The attributes in the Current Rental table were in the Game Copies table before this. But then the database is also required to record rentals history, which is why that table is also there.

    I tried to set CopiesHeld as a calculated value, but from what i could see, the only calculations i could make were using values from within the same table.

    And ok thanks i will remove Title from GameCopy and sort out the naming convention.

    Thanks again and apologies, it's a bit of a mess and i'm a little lost so trying to assist me will be quite a hassle i imagine!

  8. #8
    June7's Avatar
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    CopiesHeld would be calculated with aggregate (GROUP BY) Totals query that counts the records in GameCopy with grouping on the CatalogueNo or with DCount() domain aggregate function.

    Rentals should allow multiple records for each member. This table would be used to show if each game copy is currently rented and where as well as to summarize members rentals. This is what queries and reports are for - data manipulation.
    How to attach file: http://www.accessforums.net/showthread.php?t=70301 To provide db: copy, remove confidential data, run compact & repair, zip w/Windows Compression.

  9. #9
    nick404's Avatar
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    You can use a query to perform the operation of calculated field for CopiesHeld. A query will be able to access values from multiple tables and essentially place them into one 'table'.

  10. #10
    JJAMII is offline Novice
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    Ok thanks a lot guys.

    June, my current plan involves having a Status attribute in the GameCopies table that can be set as either Available, On loan, or Reserved. There is also a MemberID Attribute in the same table which would be left blank when available, or show the MemberID to whom the copy is on loan to or reserved by. Do you think it would be better to have the attributes such as Date Issued and Return Due, within this same table? And Remove the CurrentRentals table i recently created all together?

  11. #11
    June7's Avatar
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    Again, ideally, the status and who has the copy should be calculated in query, not saved into table.

    You might find the MS Lending Library template interesting. If I remember correctly, it shows these calculations.
    How to attach file: http://www.accessforums.net/showthread.php?t=70301 To provide db: copy, remove confidential data, run compact & repair, zip w/Windows Compression.

  12. #12
    JJAMII is offline Novice
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    Oh i see, sorry, didn't understand what you meant really the first time. Didn't know queries could be used in that way. Thanks i'll have a look!

  13. #13
    June7's Avatar
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    I will qualify my advice to say that saving the copy status is relatively simple and maybe simpler than the query approach but does require code (macro or VBA). The real trick would be figuring out what event to put code into.

    Calculating the copy count should still be done with query or DCount() and not saved to table. Saved aggregate data has risk the value will get 'out of sync' with the raw data.
    How to attach file: http://www.accessforums.net/showthread.php?t=70301 To provide db: copy, remove confidential data, run compact & repair, zip w/Windows Compression.

  14. #14
    JJAMII is offline Novice
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    I was going to have an issue rental form, from there i could enter issue date, return due and memberID of the holder, along with changing the status of the copy.

    Also once i have calculated the copy count with a query, could it then be displayed in the CopiesHeld field i mentioned earlier?

    Thanks again, will try to stop asking questions after this! Lol

  15. #15
    June7's Avatar
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    This form would be bound to Rentals table. Why would you need to show copy count?

    Could pull the calculated count from the query with DLookup() or just do a DCount() and don't build query. Always more than one way to accomplish.

    Displaying aggregate data on form can be a challenge. Reports are better suited for this.
    How to attach file: http://www.accessforums.net/showthread.php?t=70301 To provide db: copy, remove confidential data, run compact & repair, zip w/Windows Compression.

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