SQL: GROUP BY with string data
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What aggregate functions work on a string, when using a GROUP BY? 

I would usually GROUP BY user_id, but I am not sure what aggregate function can work over a string.

Goal is to get the join_date for each user_id as one row:

    user_id   join_date

    12341235  "2012-05-01"
    12341235  "2012-05-01"
    12341235  "2012-05-01"
    12341235  "2012-05-01"
    05945949  "2012-03-22"
    05945949  "2012-03-22"
    05945949  "2012-03-22"
    99910277  "2012-02-17"
    99910277  "2012-02-17"
    99910277  "2012-02-17"

Finished data:

    user_id   join_date

    12341235  "2012-05-01"
    05945949  "2012-03-22"
    99910277  "2012-02-17"