如何在 SQL 案例研究中按每个级别级别聚合？

Question

select 
Count(CASE
    WHEN quantity * saleprice <= 400 THEN 'B'
    WHEN quantity * saleprice between 400 and 999 THEN 'S'
    WHEN quantity * saleprice between 1000 and 2099 THEN 'G'
    WHEN quantity * saleprice >=2100 THEN 'D' END) AS Level,
   c.customerid CountOfCustomers
from customers c join sales s on c.customerid = s.customerid join saleitem si on s.saleid = si.saleid
group by c.customerid

我需要按此 Case 语句中的每个级别进行聚合，但是当我将整个 case 语句放在 group by 子句中时，我只能得到一行输出。

Answer 1

我怀疑您想要两个级别的聚合：首先根据客户的总销售额将客户放入存储桶，然后计算每个存储桶的客户：

select lvl, count(*) cnt_customers
from (
    select case 
        when sum(quantity * saleprice) <   400 then 'B'
        when sum(quantity * saleprice) <  1000 then 'S'
        when sum(quantity * saleprice) <  2100 then 'G'
        when sum(quantity * saleprice) >= 2100 then 'D'
    end as lvl
    from customers c 
    inner join sales s on c.customerid = s.customerid 
    inner join saleitem si on s.saleid = si.saleid
    group by c.customerid
) t
group by lvl