A database-level Login can be associated with zero to one server-instance-level Logins. And if it exists, that in turn can be associated with zero to one database-level Logins on another database.
Is it possible to retrieve the matching Login using a single query?
Given the answer to this question, I suspect it's not. But it's worth asking.
Assuming you have a local database user named foo, you can use this query to find out if there is a related user in database [splunge]:
SELECT [local].[name], [remote].[name]
FROM sys.database_principals AS [local]
INNER JOIN [splunge].sys.database_principals AS [remote]
ON [local].[sid] = [remote].[sid]
WHERE [local].[name] = 'foo';
If you don't know which other database(s) the related login may be found in, then no, there isn't a simple way without constructing a query like the answer in the other question you pointed to. If you are going to use sp_msForEachDB, please use caution:
One way to do this a little easier would be:
DECLARE @sql NVARCHAR(MAX) = N'';
SELECT @sql = @sql + '
UNION ALL SELECT ''' + QUOTENAME(name)
+ ''', name COLLATE SQL_Latin1_General_CP1_CI_AS
FROM ' + QUOTENAME(name) + '.sys.database_principals
WHERE sid IN (SELECT sid FROM x)'
FROM sys.databases
WHERE database_id > 4; -- assume ignore system dbs
SET @sql = ';WITH x AS (SELECT sid FROM sys.database_principals
WHERE name = ''foo'')' + STUFF(@sql, 1, 12, '') + ';';
PRINT @sql;
--EXEC sp_executesql @sql;
This doesn't meet your requirement of "a single query" but maybe you could explain why that is a requirement.
