Question

我在不同的表格中保存物品和订单。我想按项目计算订单。这是我的表结构。

表1：订单表

id | table2_id
1  | 1 
2  | 1 
3  | 2 
4  | 2

表2：项目表

id | user_id
1  | 1 
2  | 2 
3  | 1
4  | 2

一个项目有多个订单，如表格结构所示。现在我想把订单计算为每个项目。

我尝试过使用连接，但它给了我所有行cont：4

SELECT count(Table1.id) as order_count_for_each_item
FROM `Table2` as `t2`
LEFT OUTER JOIN `Table1` as `t2` ON `t1`.`id` = `t2`.`table2_id`;

但是我希望每个项目都计数：因为For Item1是：2而Item2是：2

结论：

正如拉胡尔建议的那样：

SELECT count(*) as order_count_for_each_item
FROM `Table2` as t2
LEFT OUTER JOIN `Table1` as t1 ON `t1`.`table2_id` = `t2`.id
GROUP BY t1.`table2_id`

这个查询给我最初想要的结果。

order_count_for_each_item
1  |
2  |

但是我接受的答案（Bibhudatta Sahoo）给了我项目计数零，以及项目标准顺序的计数。

SELECT t2.`id`,
(SELECT count(t1.id) form `Table1` as t1 
 where t2.`id` = t1.`table2_id`) as order_count_for_each_item
FROM `Table2` as t2
group by t2.`id`
order by t2.`id` asc

Item ID |  order_count_for_each_item
1       |   2
2       |   2
3       |   0
4       |   0

所以我接受了这个答案。

Answer 1

您在这里缺少group by

SELECT count(*) as order_count_for_each_item
FROM `Table2` as t2
LEFT OUTER JOIN `Table1` as t1 ON `t1`.`table2_id` = `t2`.id
GROUP BY t1.`table2_id`

Answer 2

尝试这样

SELECT t2.`id`,
(SELECT count(t1.id) form `Table1` as t1 
 where t2.`id` = t1.`table2_id`) as order_count_for_each_item
FROM `Table2` as t2
group by t2.`id`
order by t2.`id` asc

Answer 3

您必须使用aggregate函数Group by。 group您对项目的数据并获取计数。

SELECT item.id,ifnull(count(order.id),0) as order_count_for_each_item
FROM `Table2` as `item`
LEFT JOIN `Table1` as `order` ON order.table2_id=item.id
group by item.id ;

获取连接查询

结论：

3 个答案: