Question

Database design 101

Asked by: napsternova

This I am sure is basic but I am having a hard time visualizing it.  I have a Database for Tenants.  Included is tblTenant, tblState, tblDeveloper and a couple others.  Now I have hit a wall because I am trying to create a cascading combo selection using two combo boxes and I have realized I probablely didn't design this correctly.  Now, I have many tenants, many that have the same name, these tenants with the same name can be in different states and many of the same named tenants can be in the same state, There are also many different uses. each tenant can have the same or different developer.  So what is the correct way to break out this info?  Should I have a tenant table with ID, TenantName, use type table with ID, UseName, TenantID, and a developer table with ID, DeveloperName and a tenant info table with ID, TenantID, DeveloperID?  The developer can develop many different tenants in many states as well.  I am confusing myself, please help.
Thanks

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Asked On
2009-08-21 at 08:33:08ID24671708
Tags

Microsoft Access 2007 Access DB

Topics

dBase

,

Microsoft ADP

,

Access Forms

Participating Experts
3
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125
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14

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    Answers

     

    by: Helen_FeddemaPosted on 2009-08-21 at 09:06:42ID: 25153256

    Each table should have a unique ID.  I recommend using AutoNumber fields called TenantID, DeveloperID, etc., so you will know what type of record you are dealing with when you use these fields in queries.  The table of states can be a lookup table, say with the 2-letter abbreviations for storing to other tables, and the full state name for selection from a combo box.  You need tables for Tenants, Developers and maybe Use Types (I am not sure what that is; possibly it could be a lookup table).  You might need a many-to-many relationship between Tenants and Developers.  I presume one Developer could have many tenants (a one-to-many relationship), but is the reverse also possible?  Could one tenant rent or purchase from multiple developers?  If so, you need a many-to-many relationship between tenants and developers, with a linking table containing TenantID and DeveloperID, and possibly one or more fields pertaining to that particular combination of tenant and developer.

     

    by: rr_milesPosted on 2009-08-21 at 09:09:29ID: 25153277

    When designing tables as part of a system, ID fields are more efficient for computers to link a master table to other transaction tables. ID fields are useless when humans need to lookup data. So the direct answer is, yes, all tables should have an ID field that links back to your master tenant table. Based on your description, the tenant table should also have a separate state field. Several other fields may be useful for searching such as Telephone Number, Zip Code and Rental Location. You may also want to split the tenant name into first name and last name fields.  There are guidelines for database design call Rules of Normalization. You can search the Web for these. Try www.databasedesign-resource.com/normal-forms.html. Usually the first 3 rules will cover most situations. Dont go crazy with the rules, just try to find a good balance between the rule and being practical.

     

    by: pcelbaPosted on 2009-08-21 at 09:30:12ID: 25153450

    My English is not so good to know what everything the "tenant" could mean but based on the sentence "The developer can develop many different tenants in many states" I can imagine the tenant as a building or any other construction work...

    So, the Tenant cannot exist without Developer. Is it possible to have more developers for one tenant? I would suggest no for the beginning but I can imagine such situation.

    You are right, EVERY table should have its own ID as a primary key. It will allow to distinguish records easily.

    tblTenent:
    ID
    TenantName
    StateID
    UseID
    DeveloperID

    tblUse:
    ID
    UseName

    tblState:
    ID
    StateName

    tblDeveloper:
    ID
    DeveloperName
    etc.

    Even when some tenants have same names they are different and you have to distinguish them. You can use ID as unique identifier or you may add another column, e.g CODE which will also be unique to each tenant but it will contain text (up to 6 or 8 characters) and users will be able to better remember it. StateID is useful but it is a part of address and you should use it this way. The state should not be a part of some tenant unique identification. You could also add a city as the information of the same meaning.

    Above structure is suitable if each tenant has just one developer. (Relation Developer - Tenant is 1:N) If a possibility to have more developers for one tenant exists (relation M:N), then you need another table which you already named:

    tblTenantInfo:
    ID
    TenantID
    DeveloperID

    and the DeveloperID column will be removed from tblTenant because such information is stored in tblTenantInfo. BTW, I would name this table tblDeveloperTenant or tblDeveloperTenantInfo to have better idea what's stored in it.

    Above table structure are just something like rough proposal. I can imagine you'll need more information in each of them, e.g. address, contact person, contact phones, notes, etc. This could result in more tables, e.g. tblContact, tblContactType, tblDeveloperContact,  tblWhatEver...

    Good luck!

     

    by: napsternovaPosted on 2009-08-21 at 09:41:19ID: 25153547

    Wow all excellent, I do have one question reguarding Tenants that have the same name but have theie own ID (pcelba - tenants are buildings in this case).  How can I make sure the user doesn't have, "Taco Bell" (or any tenant) spelled 3 different ways?  Would just a standard ComboBox lookup on the input form work?  Thanks for clearing all of this up.  It also saved me a lot of time as it seems I was on the right tract.

     

    by: rr_milesPosted on 2009-08-21 at 10:43:56ID: 25154066

    Create a table with the unique statement.

     

    by: napsternovaPosted on 2009-08-21 at 11:03:12ID: 25154245

    rr miles,
    Not quite sure what you mean. "Create a table with the unique statement"  

    Are you saying that I SHOULD have a table for TenantID, TenantName and then my main table TenantInfo with TenantInfoID, TenantID, StateID, DeveloperID, etc, etc?

     

    by: rr_milesPosted on 2009-08-21 at 11:22:59ID: 25154404

    To you last question about a tables and structures: I would only use one table, the main one.

    My last post was referrancing:
    example mentioned "Taco Bell" could be spelled several different ways. This is a common problem when you want to run a report or query on the field. Your report ends up with several groups or queries can miss data that should be included. An additional table can be created from existing data to validate the field. Select <fields> from <some table> into <new table> unique. Cleanup the new table and the existing table. Then populate a drop box with the new table on your form. Either allow new entries on the form or perform support table maintenance somewhere else in your application.

     

    by: rr_milesPosted on 2009-08-21 at 11:28:42ID: 25154459

    To be clear: your main table should have TenantID and other fields relating to that specific tenant. There should not be any line or grid fields. These belong in a separate table.
     

     

    by: rr_milesPosted on 2009-08-21 at 11:32:09ID: 25154499

    There is no benefit to a StateID. There is a benefit to using a DeveloperID to link to the that info.

     

    by: rr_milesPosted on 2009-08-21 at 11:43:21ID: 25154578

    If the State table will hold other information about the state like the address and phone numbers of taxing agencies, etc, then you would use a separate state table. You gain nothing using 1 field to lookup 1 field in another table. Having said that, you may gain flexibility for the future. You will pay for this flexibility by trading execution speed now for ease of expandability in the future.  Everything design decision  you make is a tradeoff of features.

     

    by: Helen_FeddemaPosted on 2009-08-21 at 13:24:28ID: 25155488

    In the case of a State lookup table, sometimes it is useful to have two fields, one for the 2-letter abbreviation and one for the full state name.  Then the drop-down list can show the full name, but the abbreviation is stored in the bound field.

     

    by: Helen_FeddemaPosted on 2009-08-21 at 13:27:19ID: 25155505

    For cases of the same name, and different IDs, you need to display the name, plus other information that can let you select the correct record (state, city, office, etc.), but store the ID value for that record in the bound field.  People entering the same data in different records (for what should be the same record), but with spelling errors or typos, is a different issue, and not so easy to fix.  If you need to enter multiple records for (say) Taco Bell in different cities, you could have a combo box or listbox for selecting the franchise name, to cut down on these errors, at least in some cases.

     

    by: pcelbaPosted on 2009-08-21 at 13:47:10ID: 25155669

    The combo should contain not only the TenantName but also the state and city (the suggested unique code would also be displayed but not necessarily).  After the combo selection the full record details should appear on the form to allow the selected tenant confirmation. This will avoid many possible duplicities and input errors. Also all newly entered data should be reviewed by some responsible person to catch duplicate records at the early stage when there is not so many references yet and correction is relatively easy.

     

    by: napsternovaPosted on 2009-08-24 at 14:13:28ID: 31618901

    With three good solutions I have no choice but to split the points.

    20120131-EE-VQP-002

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