SQL find difference between two table - sql

Question

I need to compare email record (by email address) between two tables (Table A as Production data and B as old data) to find the difference and show the result in column such as "New", "Delete" and etc.
If exists in Table A, not in Table B, it should mark "New"
else if exists in Table B, not in Table A, it should mark "Delete"
if appears in both table, it should mark "Maintain"
I want the result like that
DisplayName LastName Diremail Result
==============================================
XXX XXX a#a.com New
ABC ABC 1#a.com Delete
DDD DDD 2#a.com Maintain
My code as follows:
SELECT b.DisplayName,
b.LastName,
b.diremail,
Result = CASE WHEN a.DirEmail IS NULL THEN 'New'
when b.DirEmail IS null then 'delete'
else 'Maintain'
END
FROM vHRIS_StaffDB b
LEFT JOIN HRIS_DL_Lists a
ON a.DirEmail = b.DirEmail
WHERE (
a.DirEmail IS NULL
OR a.DisplayName != b.DisplayName
)
but the data not correct as the code not return record which should "Delete"
(found in table b, not in table a)
pls advise. Thanks.

Answer 1

Sounds like you need full join instead of left join. Try this:
SELECT coalesce(b.DisplayName, a.DisplayName) DisplayName,
coalesce(b.LastName, a.LastName) LastName,
coalesce(b.diremail, a.diremail) diremail,
Result = CASE WHEN a.DirEmail IS NULL THEN 'New'
when b.DirEmail IS null then 'delete'
else 'Maintain'
END
FROM vHRIS_StaffDB b
FULL JOIN HRIS_DL_Lists a
ON a.DirEmail = b.DirEmail
WHERE (
a.DirEmail IS NULL
OR a.DisplayName != b.DisplayName
)