Select sum with input from grouped query

Question

I need to make a query that selects a grouped collection of rows from a table based on user input conditions, and then in the select i will sum data from a subset of the rows.

The setup is rather expansive to describe in a post, so here is a demostration of the problem in the simplest way i can make it:

We have this table: DemoTable

ID	StaticKey	GroupKey	Value
1	A	A	2
2	A	A	2
3	A	B	2
4	A	B	2
5	A	C	2
6	A	C	2

I make a select and groups on "StaticKey". What i would then like to do, is to, in the select clause, to select the sum of a subset of the values from the groupped data:

select 
    DT.GroupKey,       
    (select sum(D.Value) from DemoTable D where D.ID in (DT.ID) and D.GroupKey = 'A') as 'Sum of A''s',
    (select COUNT(D.ID) from DemoTable D where D.ID in (DT.ID) and D.GroupKey = 'A')  as 'Count of A''s'
from DemoTable DT
group by DT.StaticKey;

I hoped that the sum would result in a sum of 4 and a count of 2, but i get 2 and 1. So the input to the "select sum" seems to be just one id and not the collected ids. |GroupKey|Sum of A's|Count of A's| |--|--|--| |A|2|1|

If i add a group_concat of DT.ID i get them comma separated - but is it posible to get them as a collection i can use as input to the selects?

Heres sql to create the table and queries:

CREATE TABLE DemoTable
(
    ID        INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
    GroupKey  varchar(200)     null     default null,
    StaticKey varchar(200)     not null default 'A',
    Value     varchar(200)     null     default null,
    PRIMARY KEY (ID)
) ENGINE = InnoDB
  DEFAULT CHARSET = utf8;

insert into DemoTable (GroupKey, Value) values ('A', 2);
insert into DemoTable (GroupKey, Value) values ('A', 2);
insert into DemoTable (GroupKey, Value) values ('B', 2);
insert into DemoTable (GroupKey, Value) values ('B', 2);
insert into DemoTable (GroupKey, Value) values ('C', 2);
insert into DemoTable (GroupKey, Value) values ('C', 2);


select DT.GroupKey,
       (select sum(D.Value) from DemoTable D where D.ID in (DT.ID) and D.GroupKey = 'A') as 'Sum of A''s',
       (select COUNT(D.ID) from DemoTable D where D.ID in (DT.ID) and D.GroupKey = 'A')  as 'Count of A''s'
from DemoTable DT
group by DT.StaticKey;

DROP TABLE DemoTable;

Answer 1

Just use a CASE

select DT.GroupKey,
       SUM(case when GroupKey = 'A' THEN Value END) as 'Sum of A''s',
       COUNT(case when GroupKey = 'A' THEN Value END)  as 'Count of A''s'
from DemoTable DT
group by DT.StaticKey;