问题
I have a table of the form
Category Time Qty
A 1 20
B 2 3
A 3 43
A 4 20
B 5 25
I need a running total to be calculated by category in MySQL. The result would look something like this:
Category Time Qty Cat.Total
A 1 20 20
B 2 3 3
A 3 43 63
A 4 20 83
B 5 25 28
Any idea how I could do this efficiently in MySQL? I have searched far and wide, but all I can find is info on how to insert one single running total in MySQL. I wonder if there's any way to use GROUP BY or a similar construct to achieve this.
回答1:
You could calculate the sum in a subquery:
select Category
, Time
, Qty
, (
select sum(Qty)
from YourTable t2
where t1.Category = t2.Category
and t1.Time >= t2.Time
) as CatTotal
from YourTable t1
Trading readability for speed, you can use a MySQL variable to hold the running sum:
select Category
, Time
, Qty
, @sum := if(@cat = Category,@sum,0) + Qty as CatTotal
, @cat := Category
from YourTable
cross join
(select @cat := '', @sum := 0) as InitVarsAlias
order by
Category
, Time
The ordering is required for this construct to work; if you need a different order, wrap the query in a subquery:
select Category
, Time
, Qty
, CatTotal
from (
select Category
, Time
, Qty
, @sum := if(@cat = Category,@sum,0) + Qty as CatTotal
, @cat := Category
from YourTable
cross join
(select @cat := '', @sum := 0) as InitVarsAlias
order by
Category
, Time
) as SubQueryAlias
order by
Time
来源:https://stackoverflow.com/questions/5032360/running-sums-for-multiple-categories-in-mysql