Question

我设置了几张表来表示不同的产品，因为有些产品与其他产品具有不同的属性。

我正在尝试计算从特定城市列出的项目数量。我的sql语句如下：

Select count(*) as count from location, tbl_books where location.city = tbl_books.location && location.city = 'London' && location.country = 'United Kingdom'

Select count(*) as count from location, tbl_clothes where location.city = tbl_clothes.location && location.city = 'London' && location.country = 'United Kingdom'

循环计算该国内所有城市的结果。这些循环代表内循环。外部循环遍历不同的类别表。

我发现这不是很有效，并且想知道是否有人能够提出任何解决此问题的方法。我只想创建一个包含城市和类别字段的主表，只需在每次从特定城市添加新项目时增加主表中的计数。

Answer 1

以下SQL查询将count和location.city值作为一个查询返回，方法是将location table.city键与tbl_books.location键一起使用。

Select count(*) as count, location.city 
from location, tbl_books 
where location.city = tbl_books.location
group by location.city

Answer 2

这将总结所有城市的书籍，对于英国的所有城市（从我的头脑中，未经测试）：

select tbl_books.location, count(tbl_books.location) as count
from location inner join tbl_books on location.city = tbl_books.location
where location.country = 'United Kingdom'
group by tbl_books.location

结果应如下所示：

location  |  count
--------------------
London    |  5
--------------------
Newcastle |  1
--------------------
...       | ...

Answer 3

此查询应该为您提供所有城市和国家/地区的图书计数，不需要任何循环：

SELECT 
    location.country, 
    location.city,
    COUNT(*) as `count` 
FROM locaton
INNER JOIN tbl_books ON location.city = tbl_books.location 
GROUP BY location.country, location.city
ORDER BY location.country, location.city

结果将如下所示：

country          |   city    |   count
-----------------+-----------+-----------
France           | Paris     |  1
United Kingdom   | Belfast   |  2
United Kingdom   | London    |  3
...

循环内循环mysql计数

3 个答案: