You could run some traces on your MSSQL database to find out what the query being generated by Tableau looks like in terms of 'converting' your binary data. Or alternatively, you could see what the generated SQL looks like through Tableau - Viewing Underlying SQL Queries | Tableau Software
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Tableau does not currently have a proper support for binary data types.
I was looking a this issue couple months back and Tableau basically brings binary data as raw bytes, then converts (or at least tries to) bytes into characters (i.e. 48 is "0", 65 is "A") BUT 00 byte is treated as string terminator so everything is dropped after first occurrence of it.
If I recall correctly - join will be performed on database, before the conversion (i.e. should work).
p.s.s. My previous investigation was done on MySQL but I'm pretty sure logic is the same for MSSQL
Thanks Dmitry, im chasing up with support to get confirmation and will post their reply here when I get it.