每个唯一列值限制一行，按最大列值选择

Question

I'm trying to select the most recent WorkOrder for each Customer . I've used ORDER BY and GROUP BY , but the result returned is not the newest WorkOrder .

Table Structure:

WorkOrder
    id
    customer_id
    create_date
    ...

Sample Data:

WorkOrder.id    WorkOrder.customer_id   WorkOrder.create_date   ...
1               1                       2012-01-01 12:00:00
2               1                       2016-06-29 12:00:00
3               2                       2015-05-05 12:00:00

Desired Result:

WorkOrder.id    WorkOrder.customer_id   WorkOrder.create_date   ...
2               1                       2016-06-29 12:00:00
3               2                       2015-05-05 12:00:00

Current Code:

$query = DB::table('WorkOrder')
    ->orderBy('WorkOrder.create_date', 'desc')
    ->groupBy('WorkOrder.customer_id');

// select * from `WorkOrder` group by `WorkOrder`.`customer_id` order by `WorkOrder`.`create_date` desc

Current Result:

WorkOrder.id    WorkOrder.customer_id   WorkOrder.create_date   ...
1               1                       2012-01-01 12:00:00
3               2                       2015-05-05 12:00:00

Answer 1

使用最大功能

SELECT WorkOrder.id, WorkOrder.customer_id, MAX(WorkOrder.create_date) FROM `WorkOrder` GROUP BY `WorkOrder`.`customer_id` ORDER BY `WorkOrder`.`create_date` DESC

Answer 2

Try this:

SELECT * FROM WorkOrder 
WHERE (customer_id, create_date) IN 
      (SELECT customer_id, MAX(create_date) FROM WorkOrder
       GROUP By customer_id)
ORDER BY ...

Answer 3

I tend to do this with a correlated subquery:

select wo.*
from workorder wo
where create_date = (select max(wo2.create_date)
                     from workorder wo2
                     where wo2.customer_id = wo.customer_id
                    );

That advantage to this approach over using a group by in the subquery:

• If you have other filters in the where clause, then this will only look up the values on a reduced set of rows.
• It can use an index for the lookup, without having to first do an aggregation.