Question

I am currently making a program that will send an email if the date of a persons last entry was over (some time frame). my table is laid out like this:

employee   | dept. | date       | other | 
bob        | 1     | 2012-05-29 | abc   |
bob        | 1     | 2012-07-15 | xyz   |
jon        | 2     | 2012-05-29 | abc   |

(i have sorted with mysql by employee then date)
so for example, for bob i want to automatically assign a variable to the date 2012-07-15 because that is the date of his last entry. then based on the current date i want to send an email if that time in between submissions has been to long. My question is how to i assign a variable to the last date of each person in the table? I am also open to different better ways of doing this. Thank you.

Answer 1

To return the latest date for each employee, something like this will work.

SELECT employee
     , MAX(`date`) AS latest_date
  FROM mytable
 GROUP BY employee
 ORDER BY employee

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Addendum,

As simbabque points out, this works for getting the latest date, but does not return the other value. There are a couple of approaches to getting that result set.

If we are guaranteed that the (employee,date) is UNIQUE (for example. by a unique constraint), we can return other columns on the row that has the latest date with a query like this:

SELECT t.employee, t.`date`, t.other
 FROM mytable t
 JOIN ( SELECT r.employee, MAX(r.`date`) AS latest_date
         FROM mytable r
        GROUP BY r.employee
      ) s
   ON s.employee = t.employee
  AND s.latest_date = t.`date`
ORDER BY t.employee

If we aren’t guaranteed that (employee, date) is unique, this query would not suffice. But there are a couple of approaches to addressing that issue.