SQL Logic: Finding Non-Duplicates with Similar Rows - sql

Question

I'll do my best to summarize what I am having trouble with. I never used much SQL until recently.
Currently I am using SQL Server 2012 at work and have been tasked with trying to find oddities in SQL tables. Specifically, the tables contain similar information regarding servers. Kind of meta, I know. So they each share a column called "DB_NAME". After that, there are no similar columns. So I need to compare Table A and Table B and produce a list of records (servers) where a server is NOT listed in BOTH Table A and B. Additionally, this query is being ran against an exception list. I'm not 100% sure of the logic to best handle this. And while I would love to get something "extremely efficient", I am more-so looking at something that just plain works at the time being.
SELECT *
FROM (SELECT
UPPER(ta.DB_NAME) AS [DB_Name]
FROM
[CMS].[dbo].[TABLE_A] AS ta
UNION
SELECT
UPPER(tb.DB_NAME) AS [DB_Name]
FROM
[CMS].[dbo].[TABLE_B] as tb
) AS SQLresults
WHERE NOT EXISTS (
SELECT *
FROM
[CMS].[dbo].[TABLE_C_EXCEPTIONS] as tc
WHERE
SQLresults.[DB_Name] = tc.DB_NAME)
ORDER BY SQLresults.[DB_Name]

Answer 1

One method uses union all and aggregation:
select ab.*
from ((select upper(name) as name, 'A' as which
from CMS.dbo.TABLE_A
) union all
(select upper(name), 'B' as which
from CMS.dbo.TABLE_B
)
) ab
where not exists (select 1
from CMS.dbo.TABLE_C_EXCEPTION e
where upper(e.name) = ab.name
)
having count(distinct which) <> 2;
SQL Server is case-insensitive by default. I left the upper()s in the query in case your installation is case sensitive.

Answer 2

Here is another option using EXCEPT. I added a group by in each half of the union because it was not clear in your original post if DB_NAME is unique in your tables.
select DatabaseName
from
(
SELECT UPPER(ta.DB_NAME) AS DatabaseName
FROM [CMS].[dbo].[TABLE_A] AS ta
GROUP BY UPPER(ta.DB_NAME)
UNION ALL
SELECT UPPER(tb.DB_NAME) AS DatabaseName
FROM [CMS].[dbo].[TABLE_B] as tb
GROUP BY UPPER(tb.DB_NAME)
) x
group by DatabaseName
having count(*) < 2
EXCEPT
(
select DN_Name
from CMS.dbo.TABLE_C_EXCEPTION
)