使用MySQL进行复杂的SQL查询？

Question

我有一个数据库：

[business] must have a [location]
[location] may have many [business]
[location] may have many [postcode]
[postcode] may be for many [location]
[postcode] must have a [state]

我希望查询返回每个州的业务计数最高的10个位置。

为了更有意义，它可能会出来：

Western Australia
  - perth
  - bunbury
  - etc. up to 10

（即，珀斯在西澳大利亚州列出的企业最多）

Victoria
  - melbourne
  - st kilda
  - etc. up to 10

等等。

我希望在不使用UNION的情况下实现这一目标。 我有一段时间没有做复杂的SQL了，这伤了我的头。

我正在使用MySQL。

如果您具有类别，产品和订单，并且希望按每个类别的订单数来排名前十位的产品，则可以采用类似的方法。

Answer 1

此查询应该工作。

select * from (
    select a.state, a.location_id, C,
        @n:=case when @s=a.state then @n+1 else 1 end counter,
        @s:=a.state
    from (select @s:=null) b, (
        select pc.state, b.location_id, COUNT(b.location_id) C
        from postcode pc
        inner join location_postcode lp on lp.postcode_id=pc.id
        inner join business b on b.location_id=lp.location_id
        group by pc.state, b.location_id
        order by pc.state, C desc
    ) a
) c
where counter <= 10

我使用了易于理解的字段名称，假设这些表以规定的关系存在：

business M-1 location
location M-M postcode
    (expands to) => location 1-M location_postcode M-1 postcode
postcode M-1 state

查询已使用以下数据进行了测试：

create table business (location_id int);
insert into business select floor(rand()*10);
insert into business select floor(rand()*10) from business;
insert into business select floor(rand()*10) from business;
insert into business select floor(rand()*10) from business;
insert into business select floor(rand()*10) from business;
insert into business select floor(rand()*10) from business;
insert into business select floor(rand()*10) from business;
insert into business select floor(rand()*10) from business;
insert into business select floor(rand()*10) from business;
insert into business select floor(rand()*10) from business;
create table location_postcode (location_id int, postcode_id int);
insert into location_postcode select 1,1;
insert into location_postcode select 2,1;
insert into location_postcode select 3,1;
insert into location_postcode select 4,2;
insert into location_postcode select 5,1;
insert into location_postcode select 5,2;
insert into location_postcode select 6,1;
insert into location_postcode select 6,3;
insert into location_postcode select 7,1;
insert into location_postcode select 7,4;
insert into location_postcode select 8,5;
insert into location_postcode select 9,6;
insert into location_postcode select 10,7;
create table postcode (id int, state int);
insert into postcode select 1,1;
insert into postcode select 2,2;
insert into postcode select 3,3;
insert into postcode select 4,4;
insert into postcode select 5,4;
insert into postcode select 6,5;
insert into postcode select 7,5;

这并不能为每个“前10名”创建足够的记录，但是您将看到COUNTER列的排名正确。 要查看它是否适用于此较小的数据集，请首先将该过滤器留在此处

where counter <= 10

检查COUNTER列，然后将其减少为2或3，仅显示每个州的前2或3位。