use SUM with left join get me wrong result - sql

Question

So I have :
CREATE TABLE A (id INT,type int,amount int);
INSERT INTO A (id,type,amount) VALUES (1,0,25);
INSERT INTO A (id,type,amount) VALUES (2,0,25);
INSERT INTO A (id,type,amount) VALUES (3,1,10);
CREATE TABLE B (id INT,A_ID int,txt text);
INSERT INTO B (id,A_id,txt) VALUES (1,1,'abc');
INSERT INTO B (id,A_id,txt) VALUES (2,1,'def');
INSERT INTO B (id,A_id,txt) VALUES (3,2,'xxx');
I run this query:
SELECT min(A.id), SUM(A.amount), COUNT(B.id) FROM A
LEFT JOIN B ON A.id = B.A_id
GROUP BY A.type
I get :
min(A.id) SUM(A.amount) COUNT(B.id)
1 75 3
3 10 0
But I'm instead expecting to get :
min(A.id) SUM(A.amount) COUNT(B.id)
1 50 3
3 10 0
Can someone help? What is the best way to achieve this exact result ?
I want group BY type and get SUM of grouped A.amount and get count() of all B corresponding to its foreign key.
here is the repro : https://www.db-fiddle.com/f/esu13uGLcgFDpX7aEQRMJR/0 please RUN sql code.
EDIT to add more detail : I know the result is correct if I remove group by we can see
1, 50, 2
2, 25, 1
But I expect the above result, what is the best way to achieve it ? I want make SUM of a TYPE then count all B related to this groupped A

Answer 1

Just a shorter version of the solution. It counts B_IDs first in the inner query, so I need to Sum the counts in the outer query.
SELECT min(A.id), SUM(A.amount), Sum(Bid) FROM A
LEFT JOIN (select count(id) as Bid, A_id from B group by A_id) as Bcount
ON A.id = Bcount.A_id
GROUP BY A.type

Answer 2

This can happen when you SUM from an 1-N relation.
The matching records can multiply the result.
For example, when 1 records in A are joined with 2 in B it returns 2 times the amount of A before the GROUP BY. So a SUM then doubles A.amount.
A way to get around that is using sub-queries that join one-on-one.
And a COUNT DISTINCT can be used to count unique id's.
So this just a way to get the SUM of A correct.
SELECT
q1.type,
q1.min_id,
q2.amount,
COALESCE(q1.totalB, 0) as totalB
FROM
(
SELECT
A.type,
MIN(A.id) AS min_id,
COUNT(DISTINCT B.id) AS totalB
FROM A
LEFT JOIN B ON B.A_id = A.id
GROUP BY A.type
) AS q1
JOIN
(
SELECT
type,
SUM(amount) AS amount
FROM A
GROUP BY type
) AS q2 ON q2.type = q1.type
View on DB Fiddle
The SQL is tested for MySql. But it's an ANSI standard SQL that would run on almost any RDBMS, including MS Sql Server.

Answer 3

one way of doing this would be to use ROW_NUMBER():
WITH CTE AS (SELECT A.id AS Aid,
A.[type],
A.amount,
B.id AS bid,
txt,
ROW_NUMBER() OVER (PARTITION BY A.id ORDER BY B.id) AS RN
FROM A
LEFT JOIN B ON A.id = B.A_ID)
SELECT MIN(Aid) AS Min_A_ID,
SUM(CASE RN WHEN 1 THEN amount END) AS Amount,
COUNT(bid) AS BCount
FROM CTE
GROUP BY [type];
I also recommend getting rid of that text datatype and using varchar(MAX).