More efficient way to write query - sql-server

Question

I have two fields with an email address in #data table that I am trying to join on. I want it to join on rep email address and if that doesn't work, I want it to join on email. I tried running the following query:
select a.*
from #data a
join #email b on b.email=coalesce(a.rep_email_address,a.email)
where a.rep_email_address<>a.email
This however doesn't work, because in the case where a.rep_email_adress is not null but doesn't match with b.email, it will drop the record instead of taking the a.email field.
This is the work-around I found:
select a.*
from #data a
join #email b on a.email=b.email
except
select a.*
from #data a
join #email b on a.rep_email_address=b.email
union
select a.*
from #data a
join #email b on a.rep_email_address=b.email
where a.rep_email_address<>a.email
This however, is far from optimal, so I am wondering- any way to write this to perform better/look cleaner or simpler? Just to clarify- this query works (the second query), I am wondering if there is a better way to write it.
Thank you!

Answer 1

This should be much simpler. However, I also recommend you check the execution plan on this query to help you analyze if this is more optimal. [Or just compare the resulting execution times on your tables]
SELECT a.*
FROM #data a
JOIN #email b
ON (a.rep_email_address = b.email
OR a.email = b.email )
WHERE a.rep_email_address<>a.email;
# Not sure why or IF you need this where clause specifically.

Answer 2

Try it like this...
SELECT
a.*,
SomeColumn = ISNULL(e1.SomeColumn, e2.SomeColumn)
from
#data a
LEFT JOIN #email e1
ON e1.email = a.rep_email_address
LEFT JOIN #email e2
ON e2.email = a.email
AND e1.email IS NULL
WHERE
a.rep_email_address <> a.email
AND (
e1.email IS NOT NULL
OR
e2.email IS NOT NULL
);
HTH, Jason

Answer 3

SQL sever uses "Three-Valued Logic" in boolean evaluations:
NULL <> 2 --> Unknown (in your case in the where clause it will essentially become false)
NULL <> NULL --> Unknown (same as above)
a <> b --> true
In your case your original query should be:
select a.*
from #data a
join #email b on b.email=coalesce(a.rep_email_address,a.email)
where ISNULL( a.rep_email_address, '' ) <> ISNULL( a.email, '' )
If you care about performance, then try to avoid the use of functions in join predicates or WHERE conditions as this prevents SQL Server from using indexes on columns that are passed into the function.
SELECT a.*
FROM #data AS a
INNER JOIN #email AS b ON b.email = a.rep_email_address OR b.email = a.email
WHERE a.rep_email_address <> a.email OR ( a.rep_email_address IS NULL OR a.email IS NULL )
Summary:
Do not use NULLs to denote empty strings as this requires a lot of extra code to then check for NULLs