SQL Server / MS Access help understanding WHERE CLAUSE in terms of NULLS and LEFT JOIN

Question

I have the following example generated by MS Access for generating results base on table1 without matching table2 on the IP Address columns.

SELECT 
    Table1.ID, Table1.IP_Address, Table1.Field1 
FROM 
    Table1 
LEFT JOIN
    Table2 ON Table1.[IP_Address] = Table2.[IP Address] 
WHERE 
    (((Table2.[IP Address]) IS NULL)); 

While trying to analyze "WHERE (((Table2.[IP Address]) Is Null))" I do not understand how this makes sense, as I interpret it as only return results that are NULL for table2@IP Address. My understanding of WHERE clause is like a filter mechanism for your query and NULL is blank. Can someone help me understand this counter-intuitive statement?

Answer 1

You already mentioned the answer:

generating results base on table1 without matching table2

You use a LEFT JOIN , so you get all the rows from the LEFT table and matching and empty ( null ) as unmatced rows from the RIGHT table.
The unmatched rows from the RIGHT table will have Table2.[IP Address] equal to Null (since they are unmatching).
So the condition:

WHERE Table2.[IP Address] Is Null

will do exactly what you need:

fetch only these rows from the LEFT table that do not have a match on the RIGHT table

.

Answer 2

First, a more intuitive way to write the query would use NOT EXISTS :

SELECT Table1.ID, Table1.IP_Address, Table1.Field1 
FROM Table1
WHERE NOT EXISTS (SELECT 1 
                  FROM Table2 
                  WHERE Table1.[IP_Address] = Table2.[IP Address]
                 ); 

That said, the LEFT JOIN method is perfectly reasonable -- and sensible too.

LEFT JOIN keeps all the rows in the first table ( Table1 ) and matching rows in the second. If there is no match, then the Table2 columns need to be filled with a value -- and for the non-matches, that value is NULL .

The WHERE clause is keeping only these NULL values. Voila! It keeps the rows in Table1 that have no matching value in Table2 .