Comparing data between two tables without except operator when having null values in fields - sql

Question

I have this query in SQL Server which will give me the difference between two tables
SELECT * FROM dbo.emp
except
SELECT * FROM #temp1
and I get the proper result with only one record (which is correct)
But when I use a left outer join
SELECT emp.* FROM emp
LEFT JOIN #temp1 ON emp.empid = #temp1.empid
and
emp.firstname = #temp1.firstname
AND emp.lastname = #temp1.lastname
and emp.salary = #temp1.salary
and emp.dob = #temp1.dob
WHERE #temp1.empid IS NULL;
I get 39 records. Why the difference? I have mentioned all the columns in my join condition.
I know how to do this via where clause but my intention is to learn why the outer join is not working. Basically what is it that I am doing wrong
The below code works
SELECT dbo.emp.* FROM dbo.emp
JOIN #temp1 ON emp.empid = #temp1.empid
where
emp.firstname <> #temp1.firstname
or emp.lastname <> #temp1.lastname
or emp.salary <> #temp1.salary
or emp.dob <> #temp1.dob;

Answer 1

The outer join is presumably not working because some of the fields have NULL values.
You can emulate the except using union all and group by:
SELECT emp.*
FROM ((select 'emp' as which, empid, firstname, lastname, salary, dob
from emp
) union all
(select 'temp', empid, firstname, lastname, salary, dob
from #temp1
)
) t
group by empid, firstname, lastname, salary, dob
having sum(case when which = 'temp' then 1 else 0 end) = 0;
EDIT:
You can do this with a join using more complex conditions:
SELECT emp.*
FROM emp LEFT JOIN
#temp1
ON (emp.empid = #temp1.empid or coalesce(emp.empid, #temp1.empid) is null) and
(emp.firstname = #temp1.firstname or coalesce(emp.firstname, #temp1.firstname) is null) and
(emp.lastname = #temp1.lastname or coalesce(emp.lastname, #temp1.lastname) is null) and
(emp.salary = #temp1.salary or coalesce(emp.salary, #temp1.salary) is null) and
(emp.dob = #temp1.dob or or coalesce(emp.dob, #temp1.dob ) is null)
WHERE #temp1.empid IS NULL;