SQL Server : error "Must Declare the Scalar Variable" - tsql

Question

Trying to insert into a table from other two tables with a loop
DECLARE #RowCount INT
SET #RowCount = (SELECT Max(FogTopicsID) FROM FSB_FogTopics )
DECLARE #I INT
SET #I = 1
WHILE (#I <= #RowCount)
BEGIN
DECLARE #FogID INT, #StudentID INT, #TopicID INT, #ProcessStudentId INT
SELECT #FogID = FogID, #StudentID = StudentID, #TopicID = TopicsID
FROM FSB_FogTopics
WHERE FogTopicsID = #I
SELECT #ProcessStudentId = ProStudentId
FROM FSB_ProcessStudents
WHERE ProcessId = #FogID AND StudentId = #StudentID
INSERT INTO FSB_ProcessTopics( [ProcessStudentId], [TopicId])
VALUES (#ProcessStudentId, #TopicID)
SET #I = #I + 1
END
but I get an error
Must Declare the Scalar Variable #ProcessStudentId

Answer 1

As pointed out by forklift's comment - You can use proper set based solution instead of horrible loop like so;
INSERT FSB_ProcessTopics( [ProcessStudentId], [TopicId])
SELECT
s.ProStudentId,
f.TopicsId
FROM FSB_FogTopics f
INNER JOIN FSB_ProcessStudents s
ON f.FogId = s.ProcessId
AND f.StudentId = s.StudentId
While I realise this doesn't answer your question per-say, this is a better way to do it and should eliminate the need to solve your problem...
You probably have non-continuous Ids - So you have 1,2,4 as Ids but your code is trying to dind 1,2,3,4

Answer 2

You don't need loops to do this (you should almost never need to use loops in SQL for anything). You can do your INSERT in a single statement:
Insert FSB_ProcessTopics
(ProcessStudentId, TopicId)
Select P.ProStudentId, T.TopicsId
From FSB_FogTopics T
Join FSB_ProcessStudents P On P.ProcessId = T.FogId
And P.StudentId = T.StudentId

Answer 3

Do this as a single statement:
INSERT FSB_ProcessTopics(ProcessStudentId, TopicId)
SELECT ProStudentId, TopicsID
FROM FSB_FogTopics ft JOIN
FSB_ProcessStudents ps
ON ft.StudentID = ps.StudentId AND sps.ProcessId = ft.FogiId;
This should replace the cursor, the loop, everything.