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Avoiding duplication of imported data

Question asked by JG on Feb 5, 2013
Latest reply on Feb 6, 2013 by JG

Title

Avoiding duplication of imported data

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     I'm sure I'm not the first person to have this issue, but I can't seem to find the right search terms to find a solution! Any help would be appreciated.

     I have three (relevant) tables: Members, Events, and Attendance. Attendance is a join table between the other two, showing which members attended which events. This is similar to the Students/Classes examples I've seen about the place.

     Data is imported to Attendance, as this is the closest match to our existing setup: we receive a list of people who attended an event, details about their profession, and the relevant event_id (the Event record will already have been created prior to receiving this info).

     The list of people attending may be a mix of people who are brand new to our client list, or who have attended previous events and will therefore already have a record in the Members table. I want to create Members records for the new people and prevent any changes from happening to the details of existing Members. 

     TL;DR:

     - list of people added to Attendance join table
     - see if they are in Members table:
     -- if so, do nothing
     -- else, add new record in Members using data from Attendance

     How do I accomplish this?

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