SQL Server JOIN missing NULL values

Question

Suppose I had the following 2 tables:

      Table1:                                Table2:
Col1:      Col2:     Col3:             Col1:       Col2:       Col         


        

Answer 1

you can just map like that

select * from tableA a
join tableB b on isnull(a.colID,'') = isnull(b.colId,'')

Answer 2

You can be explicit about the joins:

SELECT Table1.Col1, Table1.Col2, Table1.Col3, Table2.Col4
FROM Table1 INNER JOIN
     Table2
      ON (Table1.Col1 = Table2.Col1 or Table1.Col1 is NULL and Table2.Col1 is NULL) AND
         (Table1.Col2 = Table2.Col2 or Table1.Col2 is NULL and Table2.Col2 is NULL)

In practice, I would be more likely to use coalesce() in the join condition:

SELECT Table1.Col1, Table1.Col2, Table1.Col3, Table2.Col4
FROM Table1 INNER JOIN
     Table2
     ON (coalesce(Table1.Col1, '') = coalesce(Table2.Col1, '')) AND
        (coalesce(Table1.Col2, '') = coalesce(Table2.Col2, ''))

Where '' would be a value not in either of the tables.

Just a word of caution. In most databases, using any of these constructs prevents the use of indexes.