MySQL Dense_Rank() 和 Max() 函数在同一个语句中不起作用

Question

I started with this MySQL query which worked fine:

select 
h.id,
h.name,
count(distinct c.sales_id)
from customers h join sales c on h.id=c.id
group by 1,2
order by 3 desc, 2 desc

However, now I want to add 2 additional columns in the same statement:

1. The first column should be the max value from the 'count(distinct c.sales_id)' column --> max(count(distinct c.sales_id))
2. The second column should add a dense_rank by the 'count(distinct c.sales_id)' column --> dense_rank() over (order by count(distinct c.sales_id))

I have a hard time adding these two columns as I keep getting an aggregation error with this query:

select 
h.id,
h.name,
count(distinct c.sales_id),
max(count(distinct c.sales_id)),
dense_rank() over (order by count(distinct c.sales_id))
from customers h join sales c on h.id=c.id
group by 1,2
order by 3 desc, 2 desc

Can someone help?

Answer 1

You might be very close to a solution.

In SQL, one can't max a count.
But one can max OVER a count.

So this should work in MySql 8.0

(untested on sample data)

select 
h.id,
h.name,
count(distinct c.sales_id) as total_uniq_sales,
MAX(count(distinct c.sales_id)) OVER (),
dense_rank() over (order by count(distinct c.sales_id))
from customers h 
join sales c on h.id=c.id
group by h.id, h.name
order by total_uniq_sales desc, h.name desc

The reason behind that is that the window functions are processed after the aggregation functions.