Elegantly handling 'enum-like' magic numbers in T-SQL code
I would actually recommend against tying this lookup in any way to SSMS / SQLCMD -specific features. Doing so would require that the code could only ever be executed using just those two programs. You wouldn't even be able to put any such logic into a stored procedure, even if it was executed from SSMS or SQLCMD. You also can't use SQLCMD commands in SSRS, etc.

Some options (that are completely independent of the client tool used to execute them):


If SQL Server 2019 is an option, then it's possible that the new Scalar UDF Inlining feature might resolve the issue with your Enum('Sales Doc Type', 'Order') approach.
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It's possible that an inline TVF might do the trick. You just might need to restructure those predicates in the WHERE clause to be INNER JOINs:

FROM  SOPxxxxx sod
INNER JOIN dbo.Enum('Sales Doc Type', 'Order') sdt
        ON sod.SOPTYPE = sdt.Value


Of course, the IN list is a little trickier as I think that might require a LEFT JOIN on each option and then a WHERE predicate to ensure that both of those did not result in a NULL:

FROM  dbo.POP10110 pod
LEFT JOIN dbo.Enum('PO Line State', 'Released') pls_r
        ON pod.POLNESTA = pls_r.Value
LEFT JOIN dbo.Enum('PO Line State', 'Change Order') pls_co
        ON pod.POLNESTA = pls_co.Value
WHERE  ( pls_r.Value IS NOT NULL
     OR  pls_co.Value IS NOT NULL)


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OR, using this same iTVF, you might be able to do non-correlated subqueries that might be more readable / manageable:

FROM  SOPxxxxx sod
WHERE sod.SOPTYPE = (SELECT sdt.Value FROM dbo.Enum('Sales Doc Type', 'Order') sdt)


And the IN list is at least looks better:

FROM  dbo.POP10110 pod
WHERE pod.POLNESTA IN (
                       SELECT pls_r.Value
                       FROM   dbo.Enum('PO Line State', 'Released') pls_r
                       UNION ALL
                       SELECT pls_co.Value
                       FROM   dbo.Enum('PO Line State', 'Change Order') pls_co
                      )            


Or perhaps (not sure what the query optimizer will prefer):

FROM  dbo.POP10110 pod
WHERE pod.POLNESTA IN (
           (SELECT pls_r.Value FROM dbo.Enum('PO Line State', 'Released') pls_r),
           (SELECT pls_co.Value FROM dbo.Enum('PO Line State', 'Change Order') pls_co)
                      )            


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If SQLCLR is an option, then SQLCLR scalar UDFs that do not do any data access and are marked as [SqlFunction(IsDeterministic = true)] are constant folded into the execution plan (the "Query Processing Architecture Guide" documentation currently states that SQLCLR functions cannot be folded, but that is provably wrong, so I suspect that the documentation was merely not updated when SQL Server 2012 added that ability; I have submitted a documentation update to fix that: Fix "Constant Folding" info in "Query Processing Architecture Guide").

The trick here is to read the data from the table without doing so in the SQLCLR scalar UDF so as to not prevent it from being foldable. In order to accomplish this, do the following:


Create a static readonly dictionary at the class level.
Create a static constructor for the class that uses a regular, external SqlConnection to connect to the local instance. This will select from the table and populate the static dictionary. This constructor will ensure that the dictionary is always populated, even if the assembly gets unloaded due to memory pressure, DBCC FREESYSTEMCACHE, etc. Unfortunately, the internal "context" connection cannot be used here since there is no Sql Context when the static constructor is executed (but would be great if there was!).
Mark the assembly as WITH PERMISSION_SET = EXTERNAL_ACCESS. No need to specify UNSAFE because the dictionary is "readonly". And even though it is "readonly", it can still have items added and/or removed from it. This should not pose any problems as it's a single use collection that is always the same for all users.
Decorate the static method that is exposed to T-SQL to do the lookup with:

[SqlFunction(IsDeterministic = true)]


No need to specify DataAccess or SystemDataAccess properties as those are set to None by default.
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This method will allow you to implement the Enum() approach that you had started doing. And, since you are working with a 3rd party app, you can keep things "clean" by installing the assembly into a separate database (and same would go for report procs, etc, I suppose). I have tested and verified that it still does the constant folding.
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OR (this is really option 3b): Given that this is a 3rd party app, IF it is safe to assume that the lookup / enum values won't be changing much, if ever, then you don't really need to read them from a table. You can simple have the entire dictionary hard-coded in the .NET code. In this approach, you could then keep the assembly marked as PERMISSION_SET = SAFE as there would be no need for that external SqlConnection back to the current instance.