Syntax:
UNCOMPRESS(string_to_uncompress)

Uncompresses a string compressed by the COMPRESS() function. If the
argument is not a compressed value, the result is NULL. This function
requires MySQL to have been compiled with a compression library such as
zlib. Otherwise, the return value is always NULL.

URL: https://dev.mysql.com/doc/refman/5.7/en/encryption-functions.html

