如何统计该类别的产品？

Question

如何计算该类别中的产品？

TABLE 类别

category_id 
title 
lft 
rght 
parent 
level

TABLE 产品

product_id 
category_id 
title

数据 Table 类别

|1|Electronics|1|16|0|0
|2|Televisions|2|7|1|1
|3|LCD|3|4|2|2
|4|PLASMA|5|6|2|2
|5|Players|8|15|1|1
|6|Mp3 players|9|10|5|2
|7|CD players|11|12|5|2
|8|DVD players|13|14|5|2
|9|Furniture|17|18|0|0

Table 产品

|1|4|Plasma 1
|2|4|Plasma 2
|3|4|Plasma 3
|4|4|Plasma 4
|5|4|Plasma 5
|6|3|LCD 1
|7|3|LCD 2
|8|3|LCD 3
|9|6|MP3 1
|10|6|MP3 2
|11|7|CD 1
|12|7|CD 2
|13|8|DVD 1
|14|8|DVD 2
|15|8|DVD 3

我想算这些例子。

Electronics (15)
Televisions (8)
LCD (3)
PLASMA (5)
Players (7)
Mp3 players (2)
CD players (2)
DVD players (3)
Furniture (0)

并使用

SQL

select parent.title, parent.level, count(product.title) as sum_products  
from category as node, category as parent, product  
where node.lft between parent.lft and parent.rght   
and node.category_id = product.category_id  
group by parent.title   
order by node.lft

这些结果

Electronics (15)
Televisions (8)
LCD (3)
PLASMA (5)
Players (7)
Mp3 players (2)
CD players (2)
DVD players (3)

我怎样才能像样本一样做出来？

家具 (0) 未显示在查询中。

感谢您的帮助。

select parent.title, parent.level, coalesce(count(product.title),0) as sum_products from category as node, category as parent, product where node.lft between parent.lft and parent.rght and node.category_id = product.category_id group by parent.title order by node.lft

http://i.stack.imgur.com/adJd2.png

谢谢jpw的回答

此代码完成

select 
  parent.title, 
  parent.level, 
  count(product.title) as sum_products  
from category as node
join category as parent 
  on node.lft between parent.lft and parent.rght   
left join product 
  on node.category_id = product.category_id  
group by parent.title, parent.level
order by node.lft

|       TITLE | LEVEL | SUM_PRODUCTS |
|-------------|-------|--------------|
| Electronics |     0 |           15 |
| Televisions |     1 |            8 |
|         LCD |     2 |            3 |
|      PLASMA |     2 |            5 |
|     Players |     1 |            7 |
| Mp3 players |     2 |            2 |
|  CD players |     2 |            2 |
| DVD players |     2 |            3 |
|   Furniture |     0 |            0 |

Answer 1

如果问题是空类别家具被排除在外，我认为解决方案是使用显式左连接而不是产品的隐式内部连接 table，如下所示：

select 
  parent.title, 
  parent.level, 
  count(product.title) as sum_products  
from category as node
join category as parent 
  on node.lft between parent.lft and parent.rght   
left join product 
  on node.category_id = product.category_id  
group by parent.title, parent.level
order by node.lft

Sample SQL Fiddle

此查询将给出以下输出：

|       TITLE | LEVEL | SUM_PRODUCTS |
|-------------|-------|--------------|
| Electronics |     0 |           15 |
| Televisions |     1 |            8 |
|         LCD |     2 |            3 |
|      PLASMA |     2 |            5 |
|     Players |     1 |            7 |
| Mp3 players |     2 |            2 |
|  CD players |     2 |            2 |
| DVD players |     2 |            3 |
|   Furniture |     0 |            0 |

如何统计该类别的产品？

How to count product in this category?

mysql

count

product