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  1. #1
    Join Date
    May 2002
    Posts
    28

    Unanswered: Need help understanding table

    Hello all,

    I have been given an empty database that is in the process of being populated from various sources. Eventually the db will be used in a GIS and mapped.

    My question is - could you help me understand how the following 3 tables work together. I am confused (escpecially with table 3) as how the information in entered and retrived.

    Alright:


    Table 1: Barriers

    Fields:
    Barrier_ID
    UTM_zone
    easting
    northing
    barrier_type (input from variety of sources)
    comments


    Table 2: Barrier_Type_Link

    Barrier_id
    Barrier_type_id


    Table 3:

    Barrier_type_id
    Description (this fields lists 15 possible barriers)
    acronym (acronyms for barriers)


    How does table 3 relate logically to table 1?


    I think i am most confused as the purpose of and the way table 3 is supposed to work (did not create db). It appears that it is a sort of look up table for data entry - (no forms have been created yet for this db) since it lists a selection of barriers a user could pick from. I don't understand this table.

    Please help!

    If you want, I could send you the tables

  2. #2
    Join Date
    Apr 2002
    Location
    Toronto, Canada
    Posts
    20,002
    yes, table 3 (barrier type) appears to be a lookup table

    table 2 (barrier type link) appears to be the "crossreference" table that links barriers to barrier types in a many-to-many relationship

    example:

    Barrier
    1
    2
    3

    Barrier Type Link
    1 204
    1 307
    3 811

    Barrier Type
    204
    307
    509
    811

    notice that barrier 1 is related to two barrier types, barrier 2 is related to none, and barrier 3 to only one

    barrier type 509 is related to no barriers

    typical many-to-many relationship

    rudy
    http://rudy.ca/

  3. #3
    Join Date
    May 2002
    Posts
    28

    rudy

    Yes, you are right - originally i had three tables - then on the advice of someone i dropped table 2 (link table)- because i needed a one to many relationship (a barrier can only have one type _ie" barrier can't be a culvert and a dam)

    but i am still confused - think i might go back to the way it was -i think the person who created this db (with 3 tables) probably knows more than i do

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