[英]Pretty basic SQL query involving count
Let's say I have the following schema 假设我有以下架构
Company:
-> company_id
-> company_name
Building_to_company:
-> building_id
-> company_id
So each building has its own id as well as a company id which relates it to a single company. 因此,每个建筑物都有自己的ID以及与单个公司相关的公司ID。
the following query gives two columns -- one for the company name, and then its associated buildings. 以下查询提供两列 - 一列用于公司名称,然后是其关联的建筑物。
SELECT company.company_name, building_to_company.building_id
FROM company, building_to_company
WHERE company.company_id = building_to_company.company_id;
The returned table would look something like this: 返回的表看起来像这样:
Company Name | Building Id
Smith Banking 2001
Smith Banking 0034
Smith Banking 0101
Smith Banking 4055
Reynolds 8191
TradeCo 7119
TradeCo 8510
So that's all simple enough. 所以这一切都很简单。
But I need to do something a bit different. 但我需要做一些不同的事情。 I need 2 columns.
我需要2列。 One for the company name and then on the right the number of buildings it owns.
一个用于公司名称,然后在右侧是它拥有的建筑物数量。 And then for a little extra challenge I only want to list companies with 3 or less buildings.
然后,为了一点额外的挑战,我只想列出3栋或更少建筑物的公司。
At this point the only real progress I've made is coming up with the query above. 在这一点上,我所做的唯一真正进展是提出上面的查询。 I know I some how have to use count on the building_id column and count the number of buildings associated with each company.
我知道如何在building_id列上使用count并计算与每个公司相关的建筑物数量。 And then at that point I can limit things by using something like
WHERE x < 4
然后在那时我可以通过使用像
WHERE x < 4
这样的东西限制事物
You've basically got it in words already. 你基本上已经用语言表达了它。 Assuming
company_name
is unique, all you have to add to your explanation to get it to work is a GROUP BY
clause: 假设
company_name
是唯一的,那么您必须添加到解释中才能使其工作的是GROUP BY
子句:
SELECT company.company_name, COUNT(building_to_company.building_id)
FROM company
INNER JOIN building_to_company
ON company.company_id = building_to_company.company_id
GROUP BY company.company_name
( SQL Fiddle demo of this query in action ) ( 此查询的SQL Fiddle演示实际上 )
To limit it to companies with 3 or less buildings, the key is you have to use a HAVING
clause and not WHERE
. 要将它限制在3栋或更少建筑物的公司,关键是您必须使用
HAVING
子句而不是WHERE
。 This is because you want to filter based on the results of an aggregate ( COUNT
); 这是因为你想根据聚合的结果(
COUNT
)进行过滤; simply put, WHERE
filters come before aggregation and HAVING
come after: 简单地说,
WHERE
过滤器在聚合和HAVING
之前出现:
SELECT company.company_name, COUNT(building_to_company.building_id)
FROM company
INNER JOIN building_to_company
ON company.company_id = building_to_company.company_id
GROUP BY company.company_name
HAVING COUNT(building_to_company.building_id) < 4
( SQL Fiddle demo of this query in action ) ( 此查询的SQL Fiddle演示实际上 )
I think you want something like this 我想你想要这样的东西
SELECT c.company_name, count(b.building_id)
FROM
company as c,
building_to_company as b
WHERE c.company_id = b.company_id
GROUP BY c.company_name;
Use the Below SQL-statement to find the company name with its own building count in descending order. 使用Below SQL语句以降序查找具有自己的构建计数的公司名称。
SELECT company.company_name, count(building_to_company.building_id)
FROM company (nolock), building_to_company(nolock)
WHERE company.company_id = building_to_company.company_id
group by company.company_name
having count(building_to_company.building_id)<=3
order by count(building_to_company.building_id) desc
Kindly let me know if you have any issue. 如果您有任何问题,请告诉我。 Thanks
谢谢
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