Use cursor for with 2 tables to insert rows - sql-server

Question

I have 2 tables: items and cost.
In table items, I have old item code and new item code
In table cost, I have the item code and the cost
I want to create new rows in table cost for the new item code according to the cost that already exist for the old item code.
For example:
I expect to see this result after I run the cursor in the cost table:
I try to run this, but it's been running forever and duplicates every row 100000 times:
DECLARE #item_code_old nvarchar (50)
DECLARE #item_code_new nvarchar (50)
DECLARE CostCURSOR CURSOR FOR
SELECT item_code_old, item_code_new
FROM item
WHERE company = 'AEW' AND item_code_new IS NOT NULL
OPEN CostCURSOR
FETCH NEXT FROM CostCURSOR INTO #item_code_old, #item_code_new
WHILE (##FETCH_STATUS = 0)
BEGIN
SELECT #item_code_old = item_code_old
FROM cost
WHERE company = 'AEW' AND year = 2021
INSERT INTO cost
SELECT
company,
year,
month,
#item_code_new,
unit_cost
FROM
cost
WHERE
company = 'AEW' AND year = 2021
FETCH NEXT FROM CostCURSOR INTO #item_code_old, #item_code_new
END
CLOSE CostCURSOR
DEALLOCATE CostCURSOR
What am I missing?

Answer 1

What you are doing with your variables doesn't really make sense, you fetch your next value into #item_code_old, but then at the start of each loop you then assign it a value pretty much at random:
SELECT #item_code_old = item_code_old
FROM cost
WHERE company = 'AEW'
AND year = 2021;
Then when it comes to inserting the new cost, you don't use this variable at all:
INSERT INTO cost
SELECT company, year, month, #item_code_new, unit_cost
FROM cost
WHERE company = 'AEW'
AND year = 2021;
I'd guess that you don't need this first step that reassigns #item_code_old, and you do need to apply the filter to the insert query.
With that being said, a cursor is completely unnecessary here (which you'll find is often the case with cursors), you can do this with a single insert, using something like this:
INSERT Cost (Company, Year, Month, item_code_old, unit_cost)
SELECT c.company, c.year, c.month, i.item_code_new, c.unit_cost
FROM cost AS c
INNER JOIN Item AS i
ON c.item_code_old = i.item_code_old
WHERE c.company = 'AEW'
AND c.year = 2021
AND i.item_code_new IS NOT NULL;
For the sake of completeness, your cursor solution would be something the following, but I would stress this is not a good approach. Cursors should be a last resort, and are probably something it is not even worth practising as it is not something you will find very useful. Instead of practicing cursors, instead practice trying to find the set-based approach, this will be significantly better use of your time. Anyway, rant over:
DECLARE #item_code_old NVARCHAR(50);
DECLARE #item_code_new NVARCHAR(50);
DECLARE CostCURSOR CURSOR LOCAL FAST_FORWARD FOR
SELECT item_code_old, item_code_new
FROM item
WHERE company = 'AEW'
AND item_code_new IS NOT NULL;
OPEN CostCURSOR;
FETCH NEXT FROM CostCURSOR INTO #item_code_old, #item_code_new;
WHILE(##FETCH_STATUS = 0)
BEGIN
INSERT INTO cost (Company, Year, Month, Item_Code_Old, unit_cost)
SELECT company, year, month, #item_code_new, unit_cost
FROM cost
WHERE company = 'AEW'
AND year = 2021
AND item_code_old = #item_code_old;
FETCH NEXT FROM CostCURSOR INTO #item_code_old, #item_code_new;
END;
CLOSE CostCURSOR;
DEALLOCATE CostCURSOR;