Question

I have a timeclock table with the following elements (id(pk), action, emp_id(fk), time).

How can I select the latest action if I pass the emp_id to the query?

id  emp_id  action  current_time
-----------------------------------------------
1   1   clockin     2012-01-29 21:52:38
2   1   clockout    2012-01-29 21:54:24
3   3   clock in    2012-01-30 21:10:51
4   4   clock in    2012-01-30 21:10:51
5   9   clock in    2012-01-30 21:11:05
6   10  clock in    2012-01-30 21:11:05
7   10  go to meal  2012-01-30 21:11:38
8   9   go to meal  2012-01-30 21:11:38
9   9   back        2012-01-30 21:12:53
10  3   back        2012-01-30 21:12:53
11  2   back        2012-01-30 21:12:53
12  1   back        2012-01-30 21:12:53

If I write my query like this:

SELECT MAX(ID) FROM timeclock WHERE emp_id = 1 

I get 12 which is right but if i

SELECT MAX(ID) , action FROM timeclock WHERE emp_id = 1

I get
id->12 and action -> 'clockin' instead of 'back' which is definitely not right

Answer 1

You’re missing a GROUP BY clause for your MAX() aggregate. The reason you got the correct answer 12 for your first query attempt was merely because that happens to be the greatest ID in the table while also coincidentally belonging to emp_id = 1. You would have gotten the same result for any of the emp_id values. A GROUP BY clause will sort this out.

Here is an example retrieving the whole row for the associated record:

SELECT * FROM timeclock 
WHERE id = (SELECT MAX(id) AS id FROM timeclock WHERE emp_id = 1 GROUP BY emp_id);

This can also be done with a HAVING clause, not needing the subquery:

SELECT action 
FROM timeclock 
WHERE emp_id = 1
GROUP BY emp_id
HAVING id = MAX(id);

For reference, the MySQL aggregate functions reference.