SQL 按组汇总值

Question

I have a following table person_amounts.

Person_id     Item_Amount   
 1             50
 1             10
 1             10
 1             20
 2             30
 2             40
 2             40

I want to write a query so that a new column Total is the sum of items by person. So the result would be:

 Person_id     Item_Amount   Total
 1             50            90
 1             10            90
 1             10            90
 1             20            90
 2             30            110
 2             40            110
 2             40            110

I could write something like:

select person_id, item_amount, (select sum(item_amount) from person_amounts pa 
where pa.person_id = p.person_id) Total from person_amounts p

However, the problem is that this query will sum up all the rows in my table for each person id. This will cause a problem if I add a where condition to limit the data. I am looking to have the Total column populate the sum based on the rows that are in my result. So if I add a where condition like

where item_amount >= 20

I'd like the results to show the following:

 Person_id     Item_Amount   Total
 1             50            70
 1             20            70
 2             30            110
 2             40            110
 2             40            110

Answer 1

You simply want a window function:

select person_id, item_amount,
       sum(item_amount) over (partition by person_id)
from person_amounts pa 
where item_amount >= 20;

Window functions are standard SQL supported by basically all databases.

If you want to use a subquery -- which is less performant and more verbose -- then you need to filter the results in the subquery as well as in the outer query.