Update 1 field in a table from another field in a different table (OS400, not a 1 to 1 relationship) - sql

Question

Im trying to update a field in a table from another field in a different table.
The table being updated will have multiple records that need updating from 1 match in the other table.
Example, i have a 1 million row sales history file. Those million records have aproximately 40,000 different sku codes, each row has a date and time stamp. Each sku will have multiple records in there.
I added a new field called MATCOST (material cost).
I have a second table containing SKU and the MATCOST.
So i want to stamp every line in table 1 with the corresponding SKU's MATCOST in table2. I cannot seem to achieve this when its not a 1 to 1 relationship.
This is what i have tried:
update
aulsprx3/cogtest2
set
matcost = (select Matcost from queryfiles/coskitscog where
aulsprx3/cogtest2.item99 = queryfiles/coskitscog.ITEM )
where
aulsprx3/cogtest2.item99=queryfiles/coskitscog.ITEM
But that results in the SQL error: Column qualifier or table COSKITSCOG undefined and highlighting the q in the last reference to queryfiles/coskitscog.Item
Any idea's ?
Kindest Regards
Adam
Update: This is what my tables look like in principle. 1 Table contains the sales data, the other contains the MATCOSTS for the items that were sold. I need to update the Sales Data table (COGTEST2) with the data from the COSKITCOG table. I cannot use a coalesce statement because its not a 1 to 1 relationship, most select functions i use result in the error of multiple selects. The only matching field is Item=Item99
I cant find a way of matching multiple's. In the example we would have to use 3 SQL statements and just specify the item code. But in live i have about 40,000 item codes and over a million sales data records to update. If SQL wont do it, i suppose i'd have to try write it in an RPG program but thats way beyond me for the moment.
Thanks for any help you can provide.

Answer 1

Ok this is the final SQL statement that worked. (there were actually 3 values to update)
UPDATE atst2f2/SAP20 ct
SET VAL520 = (SELECT cs.MATCOST
FROM queryfiles/coskitscog cs
WHERE cs.ITEM = ct.pnum20),
VAL620 = (SELECT cs.LABCOST
FROM queryfiles/coskitscog cs
WHERE cs.ITEM = ct.pnum20),
VAL720 = (SELECT cs.OVRCOST
FROM queryfiles/coskitscog cs
WHERE cs.ITEM = ct.pnum20),
WHERE ct.pnum20 IN (SELECT cs.ITEM
FROM queryfiles/coskitscog cs)

Answer 2

This more compact way to do the same thing should be more efficient, eh?
UPDATE atst2f2/SAP20 ct
SET (VAL520, VAL620, VAL720) =
(SELECT cs.MATCOST, cs.LABCOST, cs.OVRCOST
FROM queryfiles/coskitscog cs
WHERE cs.ITEM = ct.pnum20)
WHERE ct.pnum20 IN (SELECT cs.ITEM
FROM queryfiles/coskitscog cs)

Answer 3

Qualify the columns with correlation names.
UPDATE AULSPRX3/COGTEST2 A
SET A.matcost = (SELECT matcost
FROM QUERYFILES/COSKITSCOG B
WHERE A.item99 = B.item)
WHERE EXISTS(SELECT *
FROM QUERYFILES/COSKITSCOG C
WHERE A.item99 = C.item)

Answer 4

From UPDATE, I'd suggest:
update
aulsprx3/cogtest2
set
(matcost) = (select Matcost from queryfiles/coskitscog where
aulsprx3/cogtest2.item99 = queryfiles/coskitscog.ITEM)
where
aulsprx3/cogtest2.item99=queryfiles/coskitscog.ITEM
Note the braces around matcost.