update existing column with results of select query using sql - sql

Question

I am trying to update a column called Number_Of_Marks in our Results table using the results we get from our SELECT statement. Our select statement is used to count the numbers of marks per module in our results table. The SELECT statement works and the output is correct, which is
ResultID ModuleID cnt
-------------------------
111 ART3452 2
114 ART3452 2
115 CSC3039 3
112 CSC3039 3
113 CSC3039 3
The table in use is:
Results: ResultID, ModuleID, Number_Of_Marks
We need the results of cnt to be updated into our Number_Of_Marks column. This is our code below...
DECLARE #cnt INT
SELECT #cnt
SELECT C.cnt
FROM Results S
INNER JOIN (SELECT ModuleID, count(ModuleID) as cnt
FROM Results
GROUP BY ModuleID) C ON S.ModuleID = C.ModuleID
UPDATE Results
SET [Number_Of_Marks] = (#cnt)

Answer 1

You can do this in SQL Server using the update/join syntax:
UPDATE s
SET [Number_Of_Marks] = c.cnt
FROM Results S INNER JOIN
(SELECT ModuleID, count(ModuleID) as cnt
FROM Results
GROUP BY ModuleID
) C
ON S.ModuleID = C.ModuleID;
I assume that you want the count from the subquery, not from the uninitialized variable.
EDIT:
In general, when you change the question it is better to ask another question. Sometimes, though, the changes are really small. The revised query looks something like:
UPDATE s
SET [Number_Of_Marks] = c.cnt,
Marks = avgmarks
FROM Results S INNER JOIN
(SELECT ModuleID, count(ModuleID) as cnt, avg(marks * 1.0) as avgmarks
FROM Results
GROUP BY ModuleID
) C
ON S.ModuleID = C.ModuleID;
Note that I multiplied the marks by 1.0. This is a quick-and-dirty way to convert an integer to a numeric value. SQL Server takes averages on integers and produces an integer. Usually you want some sort of decimal or floating value.