--UTF-16LE согласно https://www.base64decode.org/
SELECT CAST(N'Вася пошел гулять 123 qaz' as varbinary(max)) FOR XML PATH(''), BINARY base64

SELECT CAST( CAST( 'EgQwBEEETwQgAD8EPgRIBDUEOwQgADMEQwQ7BE8EQgRMBCAAMQAyADMAIABxAGEAegA=' as XML ).value('.','varbinary(max)') AS nvarchar(max) )

--UTF-8 Русский не держит
SELECT CAST( CAST( 'MTIzNDU2Nzg5MCBWYXNqYSBwb3NoZWwgZ3VsYXQ=' as XML ).value('.','varbinary(max)') AS varchar(max) )