TOP Returning null - sql

Question

I have the following view below. The second nested select is always returning null when I use the TOP(1) clause, but when I remove this clause it returns the data as expected, just more rows than is needed. Does anyone see anything that would explain this?
SELECT TOP (100) PERCENT
a.ITEMID AS Model
,id.CONFIGID
,id.INVENTSITEID AS SiteId
,id.INVENTSERIALID AS Serial
,it.ITEMNAME AS Description
,CASE WHEN it.DIMGROUPID LIKE '%LR-Y' THEN 'Y'
ELSE 'N'
END AS SerialNumberReqd
,ISNULL(it.PRIMARYVENDORID, N'') AS Vendor
,ISNULL(vt.NAME, N'') AS VendorName
,id.INVENTLOCATIONID AS Warehouse
,id.WMSLOCATIONID AS Bin
,ISNULL(CONVERT(varchar(12), CASE WHEN C.DatePhysical < '1901-01-01'
THEN NULL
ELSE C.DatePhysical
END, 101), N' ') AS DeliveryDate
,CASE WHEN (a.RESERVPHYSICAL > 0
OR C.StatusIssue = 1)
AND c.TransType = 0 THEN C.PONumber
ELSE ''
END AS SoNumber
,'' AS SoDetail
,ISNULL(C.PONumber, N'') AS RefNumber
,ISNULL(CONVERT(varchar(12), CASE WHEN ins.ProdDate < '1901-01-01'
THEN NULL
ELSE ins.PRODDATE
END, 101), N' ') AS DateReceived
,it.STKSTORISGROUPID AS ProdGroup
,ISNULL(CONVERT(varchar(12), CASE WHEN ins.ProdDate < '1901-01-01'
THEN NULL
ELSE ins.PRODDATE
END, 101), N' ') AS ProductionDate
,it.ITEMGROUPID
,it.STKSTORISGROUPID AS MerchandisingGroup
,CASE WHEN a.postedValue = 0
THEN (CASE WHEN D.CostAmtPosted = 0 THEN D.CostAmtPhysical
ELSE D.CostAmtPosted
END)
ELSE a.POSTEDVALUE
END AS Cost
,CASE WHEN a.PHYSICALINVENT = 0 THEN a.Picked
ELSE a.PhysicalInvent
END AS PhysicalOnHand
,ins.STKRUGSQFT AS RugSqFt
,ins.STKRUGVENDSERIAL AS RugVendSerial
,ins.STKRUGVENDDESIGN AS RugVendDesign
,ins.STKRUGEXACTSIZE AS RugExactSize
,ins.STKRUGCOUNTRYOFORIGIN AS RugCountryOfOrigin
,ins.STKRUGQUALITYID AS RugQualityId
,ins.STKRUGCOLORID AS RugColorId
,ins.STKRUGDESIGNID AS RugDesignId
,ins.STKRUGSHAPEID AS RugShapeId
,CASE WHEN (a.AVAILPHYSICAL > 0) THEN 'Available'
WHEN (id.WMSLOCATIONID = 'NIL') THEN 'Nil'
WHEN (a.RESERVPHYSICAL > 0)
AND (c.TransType = 0) THEN 'Committed'
WHEN (a.RESERVPHYSICAL > 0) THEN 'Reserved'
WHEN (id.WMSLOCATIONID LIKE '%-Q') THEN 'Damaged'
WHEN (a.Picked > 0) THEN 'Picked'
ELSE 'UNKNOWN'
END AS Status
,'' AS ReasonCode
,'' AS BaseModel
,ISNULL(CAST(ins.STKSTORISCONFIGINFO AS nvarchar(1000)), N'') AS StorisConfigInfo
,ISNULL(C.ConfigSummary, N'') AS ConfigSummary
FROM
dbo.INVENTSUM AS a WITH (NOLOCK)
INNER JOIN dbo.INVENTDIM AS id WITH (NOLOCK)
ON id.DATAAREAID = a.DATAAREAID
AND id.INVENTDIMID = a.INVENTDIMID
LEFT OUTER JOIN dbo.INVENTTABLE AS it WITH (NOLOCK)
ON it.DATAAREAID = a.DATAAREAID
AND it.ITEMID = a.ITEMID
LEFT OUTER JOIN dbo.VENDTABLE AS vt WITH (NOLOCK)
ON vt.DATAAREAID = it.DATAAREAID
AND vt.ACCOUNTNUM = it.PRIMARYVENDORID
LEFT OUTER JOIN dbo.INVENTSERIAL AS ins WITH (NOLOCK)
ON ins.DATAAREAID = id.DATAAREAID
AND ins.INVENTSERIALID = id.INVENTSERIALID
LEFT OUTER JOIN (SELECT TOP (1)
itt.ITEMID
,invt.INVENTSERIALID
,itt.DATEPHYSICAL AS DatePhysical
,itt.TRANSREFID AS PONumber
,itt.TRANSTYPE AS TransType
,itt.STATUSISSUE AS StatusIssue
,dbo.stkRowsToColumn(itt.INVENTTRANSID, 'STI') AS ConfigSummary
,itt.RECID
FROM
dbo.INVENTTRANS AS itt WITH (NOLOCK)
INNER JOIN dbo.INVENTDIM AS invt WITH (NOLOCK)
ON invt.DATAAREAID = itt.DATAAREAID
AND invt.INVENTDIMID = itt.INVENTDIMID
WHERE
(itt.DATAAREAID = 'STI')
AND (itt.TRANSTYPE IN (0, 2, 3, 8))
AND (invt.INVENTSERIALID <> '')
ORDER BY
itt.RECID DESC) AS C
ON C.ITEMID = a.ITEMID
AND C.INVENTSERIALID = id.INVENTSERIALID
LEFT OUTER JOIN (SELECT TOP (1)
itt2.ITEMID
,invt2.INVENTSERIALID
,itt2.COSTAMOUNTPOSTED AS CostAmtPosted
,itt2.COSTAMOUNTPHYSICAL + itt2.COSTAMOUNTADJUSTMENT AS CostAmtPhysical
,itt2.RECID
FROM
dbo.INVENTTRANS AS itt2 WITH (NOLOCK)
INNER JOIN dbo.INVENTDIM AS invt2 WITH (NOLOCK)
ON invt2.DATAAREAID = itt2.DATAAREAID
AND invt2.INVENTDIMID = itt2.INVENTDIMID
WHERE
(itt2.DATAAREAID = 'STI')
AND (itt2.TRANSTYPE IN (0, 2, 3, 4, 6, 8))
AND (invt2.INVENTSERIALID <> '')
ORDER BY
itt2.RECID DESC) AS D
ON D.ITEMID = a.ITEMID
AND D.INVENTSERIALID = id.INVENTSERIALID
WHERE
(a.DATAAREAID = 'STI')
AND (a.CLOSED = 0)
AND (a.PHYSICALINVENT > 0)
AND (it.ITEMGROUPID LIKE 'FG-%'
OR it.ITEMGROUPID = 'MULTISHIP')
ORDER BY
SiteId
,Warehouse

Answer 1

Presumably, the top value in the subquery doesn't meet the subsequent join conditions. That is, this condition is not met:
D.ITEMID = a.ITEMID AND D.INVENTSERIALID = id.INVENTSERIALID
You are using a left outer join, so NULL values are filled in.
EDIT:
To re-iterate. When you run it with top 1, there are no values (for at least some combinations of the two variables). So, NULL will be filled in for these values. After all, top 1 (with or without the parentheses) returns only one row.
When you run it returning multiple rows, presumably there are matches. For the rows that match, the corresponding values are put it. This is the way that left outer join works.

Answer 2

Gordon's answer is correct as to why I was getting a few rows when removing top and none when I had it. The subquery in question was returning all the rows in the InventTrans table (5 million+) so when I used top, it was just getting the first row which didn't have anything. I realized this was the case when I was trying random high values (e.g 50000) in the TOP clause.
The ultimate fix was to change the left outer joins on the C and D subqueries to Cross Apply, and then change the where clauses to better filter the table (e.g itt.itemid = a.itemid and invt1.inventserialid = id.inventserialid). Using that, I was able to use TOP 1 as expected.