> Correct answer: This function returns a GZip compressed version of the string.
I disagree with the answer.
COMPRESS can return a GZip compressed version of the string, but it does not have to.
According to MSDN https://msdn.microsoft.com/en-us/library/cc280449.aspx
"SQL Server 2016 supports row and page compression for rowstore tables and indexes, and supports columnstore and columnstore archival compression for columnstore tables and indexes."
Also from same article,
"Data can also be compressed using the GZIP algorithm format. This is an additional step and is most suitable for compressing portions of the data when archiving old data for long term storage. "
Irregardless, good question that made me learn something new.