T-SQL Hmac (Rfc2104 SHA2_256) Implementation
Looks OK to me, without knowing Transact SQL anyway. Some things to consider:


obviously this implements just HMAC-SHA256, so the function should be named that way;
the key parameters is 8000 varbinary max value. That seems largish to me (although it might not matter much as the size is variable anyway);
there is no way to perform a hash over multiple fields in the DB which might be a use case (no incremental update possibility);
similarly, often it is required to reuse the key for multiple calculations, in that case you can cache the Pads.i and Pads.o values.


I'm also missing some constants being introduced but that might be due to the nature of TSQL.

Don't forget to test against NIST test vectors, just to be sure.