We really need to see the execution plan.  Estimated is probably good enough, although actual is better.

You need a separate table to specify a unique entry for ( [Entity Name], [Entity EIN] ).

(Actually, hopefully EIN by itself would be unique.  If it is, use just that. ...

Which specific brand of SQL?  MySQL?  SQL Server?  Oracle (not likely in a 2-person shop, too expensive)?

You can't pass expressions as parameter values.  You need to resolve the expression yourself and pass only a single (scalar) value.  For example:

DECLARE @start_date date

DECLARE @end_date date

SET @start_date = DATEADD(MONTH,...

I don't know that  you'll be able to fix it, particularly without the details of how the "standard audit dates" are maintained.

It seems like this is an integrity check to...

Right, good point.  I meant to do that.  You just do an INNER JOIN to the table of allowed values, with a WHERE clause(s) if needed .

Quite right.  If you wanted to use a trigger, you'd have to put a trigger(s) on the table(s) used within the view, not on the view itself.

Sure.  Based on the originally posted query:

SELECT @currentValue, SomeField
FROM SomeTable
WHERE @currentValue = 'Test1'
UNION ALL
--
SELECT @currentValue, SomeField
FROM AnotherTable
WHERE @currentValue = 'Test2'
UNION ALL
--
SELECT @currentValue, SomeField
FROM AThirdTable
WHERE @currentValue = 'Test3'
UNION...

Yes, I should have said "and without a loop".

For me, I'd use UNION ALL rather than a loop for something like this. SQL will have to parse all the SQL...

How would you pull data from at least 4 different tables efficiently without using a UNION ALL?

I doubt you need a temp table. You can just UNION ALL all the different query results. If a given query returns no rows, that's OK, it will...

Do you want to list inactive data in the report?

If not, you can just join to the other tables including "AccountStatus IN (1)" in the JOIN ON clause, something like...

Personally I'd use CROSS APPLY:

SELECT sl.Name, sl.Country, types.type

FROM Sales_Ledger sl

CROSS APPLY ( VALUES([Type 1]), ([Type 2]), ([Type 3]), ([Type 4]) ) AS types(type)

WHERE types.type > ''

Hopefully SQL's split algorithm isn't that poor for bit columns.

But, if it turns out to be, create an index on:

( MyTable_ID, is_MyTable_Text_NULL ).

Add a bit column to indicate null status of [MyTable_Text], then index on that column.

ALTER TABLE [dbo].[MyTable] ADD is_MyTable_Text_NULL AS CAST(CASE WHEN MyTable_Text IS NULL THEN 1 ELSE 0 END...