我必须找到每个用户,加入日期和他们在2019年作为买家发出的订单数量。
Table: Users
+----------------+---------+
| Column Name | Type |
+----------------+---------+
| user_id | int |
| join_date | date |
| favorite_brand | varchar |
+----------------+---------+
user_id是该表的主键。该表包含一个在线购物网站的用户信息,用户可以在这个网站上买卖商品。
表:订单
+---------------+---------+
| Column Name | Type |
+---------------+---------+
| order_id | int |
| order_date | date |
| item_id | int |
| buyer_id | int |
| seller_id | int |
+---------------+---------+
order_id是该表的主键。item_id是Items表的外键。buyer_id和seller_id是Users表的外键。
表:项目
+---------------+---------+
| Column Name | Type |
+---------------+---------+
| item_id | int |
| item_brand | varchar |
+---------------+---------+
item_id is the primary key of this table.
输入:Users表:
+---------+------------+----------------+
| user_id | join_date | favorite_brand |
+---------+------------+----------------+
| 1 | 2018-01-01 | Lenovo |
| 2 | 2018-02-09 | Samsung |
| 3 | 2018-01-19 | LG |
| 4 | 2018-05-21 | HP |
+---------+------------+----------------+
订单表:
+----------+------------+---------+----------+-----------+
| order_id | order_date | item_id | buyer_id | seller_id |
+----------+------------+---------+----------+-----------+
| 1 | 2019-08-01 | 4 | 1 | 2 |
| 2 | 2018-08-02 | 2 | 1 | 3 |
| 3 | 2019-08-03 | 3 | 2 | 3 |
| 4 | 2018-08-04 | 1 | 4 | 2 |
| 5 | 2018-08-04 | 1 | 3 | 4 |
| 6 | 2019-08-05 | 2 | 2 | 4 |
+----------+------------+---------+----------+-----------+
项目表:
+---------+------------+
| item_id | item_brand |
+---------+------------+
| 1 | Samsung |
| 2 | Lenovo |
| 3 | LG |
| 4 | HP |
+---------+------------+
输出:+-----------+------------+----------------+
| buyer_id | join_date | orders_in_2019 |
+-----------+------------+----------------+
| 1 | 2018-01-01 | 1 |
| 2 | 2018-02-09 | 2 |
| 3 | 2018-01-19 | 0 |
| 4 | 2018-05-21 | 0 |
+-----------+------------+----------------+
我写了下面的解决方案,但它不起作用。请说明是什么问题。
select u.user_id as buyer_id, u.join_date as join_date, count(o.order_id) as orders_in_2019
from Users u join Orders o on u.user_id = o.buyer_id
where o.order_date < date('2010-01-01') and o.order_date >= date('2019-01-01')
group by u.user_id;
结果为空。
除了日期打错(0。order_date & lt;'2010-01-01')你似乎想要所有的用户,即使他们在2019年没有订单,所以左连接和条件聚合似乎是合适的
select u.user_id as buyer_id, u.join_date as join_date
,sum(case when o.order_date < '2020-01-01' and o.order_date >= '2019-01-01' then 1 else 0 end) cnt
from users u
left join Orders o on u.user_id = o.buyer_id
group by u.user_id ;
+----------+------------+------+
| buyer_id | join_date | cnt |
+----------+------------+------+
| 1 | 2018-01-01 | 1 |
| 2 | 2018-02-09 | 2 |
| 3 | 2018-01-19 | 0 |
| 4 | 2018-05-21 | 0 |
+----------+------------+------+
4 rows in set (0.001 sec)
不能选择非该组中的直接元素
select g_user.buyer_id, Users.join_date as join_date , g_user.orders_in_2019 From (select u.user_id as buyer_id, count(o.order_id) as orders_in_2019 From Users u
join Orders o on u.user_id = o.buyer_id
where o.order_date < date('2010-01-01') and o.order_date >= date('2019-01-01')
group by u.user_id)
as g_user join Users on g_user.buyer_id = Users.user_id;
这是因为您的日期在sql:
中不正确o.order_date < date('2010-01-01')
很明显没有您的订单是在2010年之前。你应该改成
o.order_date < date('2020-01-01')