Create enumeration and use it in aggregate function - sql

Question

Is it possible to create some enumaration for 'a','b','test','123','blabla' in following statement?
sum(case when col1 in ('a','b','test','123','blabla') then col2 end) as sum
I've tried to read it from letters_table like this:
sum(case when col1 in (select letter from letters_table) then col2 end) as sum
but it told me Cannot perform an aggregate function on an expression containing an aggregate or a subquery.
and following is not working fine for me:
DECLARE #letters varchar(10)
select #letters = letter FROM letters_table
sum(case when col1 in (#letters) then col2 end) as sum
because when I print the #letters, there is just the last one 'blabla'

Answer 1

If you have col1 (a text column) and col2 (a numeric column) in a table, say myTable and letter (a text column) in another table, say letters_table, then you can use a JOIN, as below:
SELECT
--mt.col1, Include for totals by col1
SUM(mt.col2)
FROM myTable mt
INNER JOIN letters_table lt
ON mt.col1 = lt.letter;
-- GROUP BY mt.col1 Include for totals by col1
-- ORDER BY mt.col1; Include for totals by col1

Answer 2

Your second example using the subquery is probably the best - you just need to do the subquery and aggregation in two separate phases, like this:
select sum(x.col2) as sum
from (
select case
when col1 in (select letter from letters_table) then col2
else 0
end as col2
from YourTableName
) x
This should work as intended, and make the parser happy.

Answer 3

...or you could use a Common Table Expression, for example:
DECLARE #Test TABLE (col1 VARCHAR(1), col2 INT);
INSERT INTO #Test VALUES ('a', 1);
INSERT INTO #Test VALUES ('c', 20);
INSERT INTO #Test VALUES ('z', 3);
DECLARE #Letters TABLE (Letter VARCHAR(1));
INSERT INTO #Letters VALUES ('a');
INSERT INTO #Letters VALUES ('b');
INSERT INTO #Letters VALUES ('c');
WITH PreQuery AS (
SELECT
col1,
CASE WHEN l.Letter IS NULL THEN 0 ELSE 1 END AS GoodLetter,
col2
FROM
#Test t
LEFT JOIN #Letters l ON l.Letter = t.col1)
SELECT
SUM(CASE WHEN GoodLetter = 1 THEN col2 END)
FROM
PreQuery;

Answer 4

The solution posted by Richard should do the trick, but there is no need to use cte
you can simply use the code below
select
isnull(sum(mt.col2),0)
from
myTbl mt
inner join lettersTbl lt
on mt.col1 = lt.letter
If you want to include the letters involved in the results, you may use a variable. Looking at your previous post here is a code with the same sample data that you have provided
declare #yourTbl table (col1 char(1), col2 int);
insert into #yourTbl values ('a', 10)
,('b', 5)
,('c', 15)
,('d', 2)
,('a', 3)
,('b', 6)
,('c', 8)
,('d', 10);
declare #lettersTbl table (letter char(1));
-- insert your letters here
insert into #lettersTbl values ('a'),('b')
-- only needed if you want to display the letters involved too
declare #checkedLetters as varchar(50)
select #checkedLetters = concat(#checkedletters,lt.letter)
from
#letterstbl lt
select
#checkedletters
,isnull(sum(mt.col2),0)
from
#yourtbl mt
inner join #letterstbl lt
on mt.col1 = lt.letter