检查这段mysql语法错误set @n1= ceiling(0.25*(select count(*) from customers)); select @n1; set @n2=ceiling(0.8*(select count(*) from customers)); select @n2; with tmp as( select customerid,sum(amount) amt from customers join orders using(customerid) join orderitems using(orderid) group by customerid),tmp2 as( select *,rank() over(order by amt desc) num from tmp),tmp3 as( select *, case when num<@n1 then'A' when num>@n2 then'B' else 'C' end as'category' from tmp2) select *,row_number()over( partition by category order by amt desc) num1 from tmp3;

这段 MySQL 语法的确存在语法错误，是因为在 `tmp3` 子查询中给 `case` 语句的第二个 `when` 子句加了引号，导致 MySQL 无法正确解析该语句。正确的语法如下所示：

set @n1= ceiling(0.25*(select count(*) from customers)); 
select @n1; 
set @n2=ceiling(0.8*(select count(*) from customers)); 
select @n2; 
with tmp as( 
    select customerid,sum(amount) amt 
    from customers join orders using(customerid) join orderitems using(orderid) 
    group by customerid
),
tmp2 as( 
    select *,rank() over(order by amt desc) num 
    from tmp
),
tmp3 as( 
    select *, 
        case  
            when num<@n1 then 'A' 
            when num>@n2 then 'B' 
            else 'C'  
        end as category 
    from tmp2
) 
select *,row_number()over( partition by category order by amt desc) num1 
from tmp3;

注意，上面的 SQL 语句可能并不适用于所有版本的 MySQL 数据库，具体实现上可能会有所不同。