SQL Select Distinct - sql

Question

Working on a query here but I'm getting multiple Sub Items in my Sub Item Column. I want to adjust my query to only show a single Subitem. The PO table that I have may or may not have multiple subitems and thats why its showing sometimes many subitems.
SELECT
dbo.MasterTable.StartItem,
dbo.MasterTable.SubItem,
dbo.MasterTable.STDCOST,
ISNULL(dbo.PO_2_months.[Purchase Price], dbo.MasterTable.STDCOST) AS NewCost,
dbo.PO_2_months.[Purchase Price]
FROM dbo.MasterTable LEFT OUTER JOIN
dbo.PO_2_months ON dbo.MasterTable.SubItem = dbo.PO_2_months.Item
GROUP BY
dbo.MasterTable.SubItem,
dbo.MasterTable.STDCOST,
ISNULL(dbo.PO_2_months.[Purchase Price], dbo.MasterTable.STDCOST),
dbo.MasterTable.StartItem,
dbo.PO_2_months.[Purchase Price]
HAVING (dbo.MasterTable.StartItem = 'FO6534')

Answer 1

Each result in that list is distinct. There may be duplicates of some fields, but there are no two exactly identical records in that result set.
So, if you only want one result for sub item 0S331500G0, for instance, you need to specify what record you want to keep, and which ones you want to discard. Do you want the 0S331500G0 item in your result set to report a STDCOST of 0.6295 or 0.6470? Or the average thereof?
As long as you specify that, you will be able to select distinct:
SELECT DISTINCT
dbo.MasterTable.StartItem,
dbo.MasterTable.SubItem,
AVG(dbo.MasterTable.STDCOST),
AVG(dbo.MasterTable.NewCost),
AVG(dbo.MasterTable.PurchasePrice)
...
MIN(), MAX() and SUM() are other examples of Aggregate Functions you may want to use to specify how the data is to be handled where items are not distinct.

Answer 2

SELECT DISTINCT won't have any effect over what you already have. The reason being that your subitems, despite having the same ID, aren't distinct... You need to make a call on how you choose which subitem is the subitem. For example, an easy option would be to take the maximum stdcost, newcost and Purchase Price (using the MAX aggregation function); but "easy" doesn't necessarily mean "correct"!

Answer 3

It depends on ow you want to determine which of the many sub-items you want to select.
In your case, your MasterTable already has the 1:many relationship between StartItem and SubItem. This means that you could simply pre-process that table and work from there...
SELECT
dbo.MasterTable.StartItem,
dbo.MasterTable.SubItem,
dbo.MasterTable.STDCOST,
ISNULL(dbo.PO_2_months.[Purchase Price], dbo.MasterTable.STDCOST) AS NewCost,
dbo.PO_2_months.[Purchase Price]
FROM
(
SELECT StartItem, MIN(SubItem) FROM dbo.MasterTable GROUP BY StartItem WHERE StartItem =
'FO6534'
)
AS reduced_MasterTable
INNER JOIN
dbo.MasterTable
ON MasterTable.StartItem = reduced_MasterTable.StartItem
AND MasterTable.SubItem = reduced_MasterTable.SubItem
LEFT OUTER JOIN
dbo.PO_2_months
ON dbo.MasterTable.SubItem = dbo.PO_2_months.Item
This assumes a 1:1 relationship between the records in MasterTable and PO_2_Months. You'll want to put the GROUP BY back in if it's 1:many, and probably use SUM() or MAX() or something on dbo.PO_2_months.[Purchase Price].