Question

I have a system that allows users to assign a specific file to a past or present date. The limitations are that they may only upload one file per day per user. When the user goes to upload a file the date field must default to the current date and when that date is not available it will show the first available date in the past in DESC order. Below is the relevent field names.

file_id (INT - INDEX - AUTO INCREMENT)
user_id (INT - may index this)
upload_date (INT - stores date as a unix timestamp)

The only solution I have really found would be to build them all into an array in DESC order by date and loop through until i found an empty slot. However, I feel this could really cause speed issues if the user had the past thousand days filled. I feel like I am overlooking a simple solution.

PLEASE NOTE: For one reason or another they Date is being stored as a Unix timestamp which I understand the downsides on and I am not concerned about correcting at this time.

Answer 1

To get the most recent date that has not been used:

select user_id, max(date) - 1
from (select ud.*,
             (select max(date) from upload_date ud2 where ud2.user_id = ud.user_id and ud2.date < ud.date
            ) as prevdate
      from upload_date ud
     ) ud
where date(from_unixtime(ud.prevdate)) <>  date(from_unixtime(ud.date)) - 1 or
      ud.prevdate is null
group by user_id

This query first gets the previous date for any given day using a correlated subquery. It then converts the time values to dates and selects any row where the previous date has a gap. The largest of the date minus one is the date you are looking for.

This SQL is untested, so it may have syntax errors.