Update Table From Select - sql

Question

I am using ms-sql server. I have table which I want to update from select statement. For example the table which I want to update is Table_A with 2 rows in it. The update statement from which I want to update Table_A return 10 rows. So I want to update Table_A 10 times. The problem is that Table_A is updated 2 times(the count of rows in Table_A).
Example:
CREATE TABLE #tmp
(
AccountID INT,
Inflow DECIMAL(10,2)
)
DECLARE #n INT = 0
WHILE (#n <10 )
BEGIN
INSERT INTO #tmp SELECT 2, 10
SET #n += 1
END
UPDATE dbo.Table_A
SET Balance += sss.Inflow
FROM ( SELECT t.AccountID ,
t.Inflow
FROM #tmp AS t
) AS sss
WHERE dbo.tAccount.AccountID = sss.AccountID;
-- Updates only 2 times
-- What I expected here is Table_A to be updated as many times as the count of the select statement which is 10, based on the insert before.

Answer 1

Your expectation is wrong. Admittedly, the documentation buries this idea:
The example runs without error, but each SalesYTD value is updated
with only one sale, regardless of how many sales actually occurred on
that day. This is because a single UPDATE statement never updates the
same row two times.
The documentation continues with the solution:
In the situation in which more than one sale for a specified
salesperson can occur on the same day, all the sales for each sales
person must be aggregated together within the UPDATE statement, as
shown in the following example:
So, simply aggregate before doing the join:
UPDATE dbo.Table_A
SET Balance += sss.Inflow
FROM (SELECT t.AccountID, SUM(t.Inflow) as Inflow
FROM #tmp t
GROUP BY t.AccountId
) sss
WHERE dbo.tAccount.AccountID = sss.AccountID;
Note you can also write this as:
UPDATE a
SET Balance += sss.Inflow
FROM dbo.Table_A a JOIN
(SELECT t.AccountID, SUM(t.Inflow) as Inflow
FROM #tmp t
GROUP BY t.AccountId
) sss
ON a.AccountID = sss.AccountID;
This makes the JOIN more explicit.