MySQL: Get the minimum record for a user on a given day

Question

I have a table of events, each with someone in charge. There may be multiple of these events per day, but I need a query record of the first for each user on a given day.

For example, if I have the following table of events:

+----------+-------------+---------------------+
| event_id | director_id | event_start         |
+----------+-------------+---------------------+
|        1 |         111 | 2015-04-27 10:00:00 |
+----------+-------------+---------------------+
|        2 |         222 | 2015-04-27 11:00:00 |
+----------+-------------+---------------------+
|        3 |         333 | 2015-04-27 12:00:00 |
+----------+-------------+---------------------+
|        4 |         111 | 2015-04-27 13:00:00 |
+----------+-------------+---------------------+
|        5 |         222 | 2015-04-27 09:00:00 |
+----------+-------------+---------------------+

I would like the following returned:

+----------+-------------+---------------------+
| event_id | director_id | event_start         |
+----------+-------------+---------------------+
|        1 |         111 | 2015-04-27 10:00:00 |
+----------+-------------+---------------------+
|        5 |         222 | 2015-04-27 09:00:00 |
+----------+-------------+---------------------+
|        3 |         333 | 2015-04-27 12:00:00 |
+----------+-------------+---------------------+

I thought a query like the following would have worked, but it turns out that MySQL does not support MIN in the WHERE clause ( simple SQL query giving Invalid use of group function ):

SELECT
        event_id, director_id, MIN(event_start) AS event_start 
    FROM events 
    WHERE MIN(event_start) >= '2015-04-27 00:00:00' 
        AND MIN(event_start) < '2015-04-28 00:00:00'
    GROUP BY director_id;

How can I do this in the most efficient way possible? My events table may easily have 10,000-100,000 records.

Answer 1

You can get the minimum event time on each day with a query similar to yours:

SELECT director_id, date(event_start) as dte, MIN(event_start) AS event_start 
FROM events e
GROUP BY director_id, date(event_start);

You can then use this as a subquery to get all other information from the row:

select e.*
from events e join
     (SELECT e.director_id, date(e.event_start) as dte, MIN(e.event_start) AS event_start 
      FROM events e
      GROUP BY e.director_id, date(e.event_start)
     ) ee
     on e.event_start = ee.event_start -- note, this has both the date and time;

If you want to restrict the results to a single day, you can put the where clause in the subquery.

Answer 2

You cant use group by / aggregate functions in the where clause of a query. One way to do what you want is to use a left join like so:

select e1.*
  from events e1
    left join events e2
      on e1.director_id = e2.director_id
        and e1.event_start > e2.event_start
        and date(e1.event_start) = date(e2.event_start)
  where e2.director_id is null

fiddle here

Performance is likely to be increased if you have an index across (director_id, event_start)

You can also further limit the result size by changing and date(e1.event_start) = date(e2.event_start) to check for specific dates.

Answer 3

You can give this a try:

SELECT
  e1.* 
FROM events AS e1
  INNER JOIN ( SELECT director_id, MIN(event_start) AS `eventStart` 
               FROM `events` GROUP BY director_id ) AS e2
  ON e1.director_id = e2.director_id
  AND e1.event_start = e2.eventStart
WHERE e2.eventStart >= '2015-04-27 00:00:00' 
  AND e2.eventStart < '2015-04-28 00:00:00';

Here is the sqlfiddle .