如何在SQL的案例研究中按每个级别进行聚合？

在这个Case语句中，我需要按每个级别进行聚合，但是当我将整个Case语句放入group by子句中时，我只得到一行输出。

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

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

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请提供样本数据和预期结果。现在还不清楚你所说的每个级别的聚合是什么意思。谢谢！你能解释一下你使用的内部连接吗？

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