Joining 2 tables with sql - sql

Question

I am trying to join these 2 tables based on the left table and matching the row values. The right table does not necessarily have all ITEMNUMs that are in the left table. And not every VNDITNUM is going to have an ITEMNUM that is in the left hand table.
LEFT TABLE (559 rows)
--Filtered based on column IVCOGSIX = 137
SELECT
ITEMNMBR
FROM dbo.IV00101
WHERE IVCOGSIX = 137
+---------+
| ITEMNUM |
+---------+
| Item 1 |
| Item 2 |
| Item 3 |
| Item 4 |
+---------+
RIGHT TABLE (553 rows)
--Filtered based on column VENDORID = 90505EP
SELECT
ITEMNMBR,
VNDITNUM
FROM dbo.IV00103
WHERE VENDORID = '90505EP'
+---------+-------------+
| ITEMNUM | VNDITNUM |
+---------+-------------+
| Item 1 | VendorNum 1 |
| Item 2 | VendorNum 2 |
| Item 4 | VendorNum 4 |
| Item X | VendorNum 5 |
+---------+-------------+
The output that I am trying to get is this -
JOINED TABLE (559 rows)
+---------+-------------+
| ITEMNUM | VNDITNUM |
+---------+-------------+
| Item 1 | VendorNum 1 |
| Item 2 | VendorNum 2 |
| Item 3 | NULL |
| Item 4 | VendorNum 4 |
+---------+-------------+
However, my query keeps giving me only 548 rows, and I'm not necessarily sure what rows it is leaving out.
Here is my current query -
SELECT
cogs.ITEMNMBR,
vin.VNDITNUM
FROM METRO.dbo.IV00101 cogs
LEFT JOIN METRO.dbo.IV00103 vin
on vin.ITEMNMBR = cogs.ITEMNMBR
WHERE
cogs.IVCOGSIX = 137
AND vin.VENDORID = '90505EP'
How can I join these 2 tables to get the 559 rows of the left table and any matching items from the right table?

Answer 1

Your join predicate vin.VENDORID = '90505EP' has effectively turned your left join into an inner join.
Try this instead.
SELECT
cogs.ITEMNMBR,
vin.VNDITNUM
FROM METRO.dbo.IV00101 cogs
LEFT JOIN METRO.dbo.IV00103 vin
on vin.ITEMNMBR = cogs.ITEMNMBR
AND vin.VENDORID = '90505EP'
WHERE
cogs.IVCOGSIX = 137

Answer 2

To explain what happened, essentially SQL Server will join the tables together first, and then run the where clause on the results (it's a bit more complicated then that, but this is the general way it works). The WHERE clause as you originally ran it will remove anything that doesn't have vin.VENDORID = '90505EP', essentially stripping out the nulls from the result set. and returning the results of an inner join.
#SeanLange's answer works around this by shifting the filter on VENDORID into the join clause, essentially saying "filter dbo.IV00103 where VENDORID = '90505EP and then left outer join on that result"
I call (and have seen it called this before) as an Accidental Inner Join.

Answer 3

You need a Left Outer Join, so the Left table will output the full content:
SELECT
cogs.ITEMNMBR,
vin.VNDITNUM
FROM METRO.dbo.IV00101 cogs
LEFT OUTER JOIN METRO.dbo.IV00103 vin
on vin.ITEMNMBR = cogs.ITEMNMBR
WHERE
cogs.IVCOGSIX = 137
AND vin.VENDORID = '90505EP'