Unexpected NULLs in JOIN query - sql

Question

I am using 3 tables in the query: 2 with INNER JOIN and the 3rd with LEFT JOIN.
However, I am getting some NULLrecords while the LEFT JOIN condition is executing. There are coming from the Purchase line(PL_ TABLE)`which should not be the case.
Can any one recommend the change in the query to replace NULL values in the record with actual values?
Sorry, I could not find any option to attach the sample table data.
The Query
SELECT
pih.[PO Number],
pih.[Pre-Assigned No_] as [Invoice No],
pil.[Document No_],
pil.[Description] as [Reason For Discrepency],
pil.[Line No_],
pl.[No_] as [Item No],
pl.[Vendor Item No_],
pl.[Order Date],
pil.[Posting Date],
pil.[Expected Receipt Date],
pih.[Notes] as [Header Notes],
pil.[No_] as [G/L Account No],
pih.[Buy-from Vendor No_],
pih.[Buy-from Vendor Name],
Pil.Quantity as [Inv Qty From InvoiceLine],
pil.[Amount Including VAT] as [Inv Value From InvoiceLine],
pl.Quantity as [PO Quantity From Purchaseline],
pl.[Quantity Received] as [Received Qty From PurchaseLine],
pl.[Quantity Invoiced] as [Invoiced Qty From PurchaseLine] ,
pl.[Amount Including VAT] as [PO Value From PurchaseLine]
FROM
[Purch_ Inv_ Line] pil
INNER JOIN Purch_ Inv_ Header] pih
ON pil.[Document No_] = pih.[No_]
LEFT JOIN [Purchase Line] pl
ON pih.[PO Number]=pl.[Document No_] and pl.[Line No_]=pil.[Line No_]
WHERE
PIL.[Document No_] IN
(
SELECT distinct pil.[Document No_] FROM
Purch_ Inv_ Line] pil
WHERE piL.[No_] in ('700xxx','700xxx','17xxxxx') and pil.[Posting Date] >=getdate()-7
)
AND piL.[Type]='1'

Answer 1

There might be 2 reasons why you are getting NULL values:
Values of columns in [Purchase line] table are actually NULL
There are no matching rows in [Purchase line] table for rows that are NULL. This is how LEFT JOIN works
If you want to get only matching rows, then change LEFT JOIN to INNER JOIN - however then your result set will not contain rows from [Purch_ Inv_ Line] and [Purch_ Inv_ Header] where there are not matching [Purchase line] rows.
If you want to give default values to NULL values you can do it using ISNULL:
ISNULL(pl.[Amount Including VAT], 0) as [PO Value From PurchaseLine]