每个唯一列值限制一行，按最大列值选择

Question

我正在尝试为每个Customer选择最新的WorkOrder 。 我使用过ORDER BY和GROUP BY ，但是返回的结果不是最新的WorkOrder 。

表结构：

WorkOrder
    id
    customer_id
    create_date
    ...

样本数据：

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

所需结果：

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

当前代码：

$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

当前结果：

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

尝试这个：

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

我倾向于使用相关的子查询来做到这一点：

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

与在子查询中使用group by相比，该方法的优势在于：

• 如果where子句中有其他过滤器，则只会在减少的行集中查找值。
• 它可以使用索引进行查找，而不必首先进行聚合。