SQL Server: is this a bug or do I have a misunderstanding? - sql

Question

Today I'm found a very sticky problem on SQL Server 2014.
Scenario: I want to pay awards to my customer (some pin code for cell phone operator)
In last cycle of loop T.Used = 0 condition is bypassed and is not working. I know in other conditions in that query (T.Cash < (#myAwards - #paid)) is there a mistake and I must to use T.Cash <= (#myAwards - #paid) instead of this but please focus on main question.
Why it's happened when I update Used flag to 1 (True) then in next loop it's selected while it doesn't have a valid condition (T.Used = 0)?
DECLARE #myAwards INT = 90000,
#paid INT = 0;
DECLARE #Temp TABLE
(
Id INT NOT NULL,
Pin VARCHAR(100) NOT NULL,
Cash INT NOT NULL,
[Weight] INT NULL,
Used BIT NOT NULL
)
INSERT INTO #Temp
SELECT
UPFI.Id, UPFI.PinCode,
PT.Cash, NULL, 0
FROM
dbo.UploadedPinFactorItem UPFI WITH (NOLOCK)
INNER JOIN
dbo.PinType PT WITH (NOLOCK) ON PT.ID = UPFI.PinTypeID
WHERE
PT.Cash <= #myAwards
UPDATE T
SET [Weight] = ISNULL((SELECT COUNT(TT.Id)
FROM #Temp TT
WHERE TT.Cash = T.Cash), 0) * T.Cash
FROM #Temp T
--For debug (first picture)
SELECT * FROM #Temp
DECLARE #i int = 1
DECLARE #count int = 0
SELECT #count = COUNT([Id]) FROM #Temp C WHERE C.Used = 0
WHILE (#i <= #count AND #paid < #myAwards)
BEGIN
DECLARE #nextId INT,
#nextCash INT,
#nextFlag BIT;
-- 'T.Used = 0' condition is by passed
SELECT TOP (1)
#nextId = T.Id, #nextCash = T.Cash, #nextFlag = T.Used
FROM
#Temp T
WHERE
T.Used = 0
AND T.Cash < (#myAwards - #paid)
ORDER BY
T.[Weight] DESC, T.Cash DESC, T.Id DESC
UPDATE #Temp
SET Used = 1
WHERE Id = #nextId
SET #i = #i + 1
SET #paid = #paid + #nextCash
--Show result in second picture
SELECT
#i AS 'i', #paid AS 'paid', #nextFlag AS 'flag', #nextId AS 'marked Id',*
FROM
#temp T
ORDER BY
T.[Weight] DESC, T.Cash DESC, T.Id DESC
END
SELECT 'final', #paid, *
FROM #temp T
ORDER BY T.[Weight] DESC, T.Cash DESC, T.Id DESC
Please let me to understand this is a bug or I have misunderstanding
First screenshot:
Second screenshot (result of loop):
Third screenshot (final result):

Answer 1

As per my comments:
This isn't a problem with the condition, the problem is with the implemented logic. After i = 4, there are no more rows where T.Used = 0 AND T.Cash < (#myAwards - #paid), that makes it so your reassigning variables gets zero rows, so they mantain the previous values.
You can test this behavior by doing:
DECLARE #A INT = 10;
SELECT #A = object_id
FROM sys.all_objects
WHERE name = 'an object that doesn''t exist'
SELECT #A;