我需要获取上个月的客户ID，数量和订单数量。
Paper::select('customer_id', DB::raw('SUM(price) as `sum`')
,DB::raw('COUNT(*) as `orders_per_month`'))
->where('created_at', '>=', Carbon::now()->startOfMonth())
->groupBy('customer_id')->get()
我尝试过，但是如果客户上个月没有订单，则不会选择它
最佳答案
尝试COALESCE：
DB::raw('COALESCE(COUNT(*), 0) as `orders_per_month`')
此功能允许您定义默认值，否则为NULL。
http://dev.mysql.com/doc/refman/5.7/en/comparison-operators.html#function_coalesce
更新：
我认为，如果您只选择上个月有订单的客户，那么绝对没有订单就不会吸引那些客户。
您可以通过以下方式实现所需的功能：
mysql> select * from cu;
+----+----------+
| id | name |
+----+----------+
| 1 | Google |
| 2 | Yahoo |
| 3 | Mirosoft |
+----+----------+
3 rows in set (0.00 sec)
mysql> select * from so;
+----+-------------+---------------------+-------+
| id | customer_id | created_at | price |
+----+-------------+---------------------+-------+
| 1 | 1 | 2016-08-23 12:12:12 | 2 |
| 2 | 1 | 2016-09-24 12:14:13 | 3 |
| 3 | 2 | 2016-09-25 00:00:00 | 5 |
| 4 | 2 | 2016-09-12 09:00:00 | 3 |
+----+-------------+---------------------+-------+
4 rows in set (0.00 sec)
mysql> select cu.id as customer_id, coalesce(sum(so.price),0) as total, coalesce(count(so.customer_id),0) as orders_per_month from cu left join so on (cu.id = so.customer_id) where so.created_at >= '2016-09-01 00:00:00' or so.created_at is null group by cu.id;
+-------------+-------+------------------+
| customer_id | total | orders_per_month |
+-------------+-------+------------------+
| 1 | 3 | 1 |
| 2 | 8 | 2 |
| 3 | 0 | 0 |
+-------------+-------+------------------+
3 rows in set (0.00 sec)
但是，结果查询不是一种高效的方法，您将不得不扫描所有客户并加入以获得没有订单的客户。分别获取有订单和无订单的客户列表，然后将它们与UNION或通过您的应用程序代码合并，可能会更快。
mysql> select customer_id, sum(total) as total, sum(orders_per_month) as orders_per_month from (select id as customer_id, 0 as total, 0 as orders_per_month from cu union all select customer_id, sum(so.price) as total, count(so.customer_id) as orders_per_month from so where created_at >= '2016-09-01 00:00:00'group by customer_id) agg group by customer_id;
+-------------+-------+------------------+
| customer_id | total | orders_per_month |
+-------------+-------+------------------+
| 1 | 3 | 1 |
| 2 | 8 | 2 |
| 3 | 0 | 0 |
+-------------+-------+------------------+
3 rows in set (0.00 sec)