Ugh, I just wasted time debugging a MySQL script only to find that I had created a syntactically invalid comment. So, as a future reminder to myself (and to warn others), MySQL supports 3 different comment styles:
# marks the start of a comment (until the end of the line)
— also marks the start of a comment (until the end of a line)
/* */ can be used to mark the start and end of comments (supporting multi-line comment, too).
# ## and even ######### are all valid comment designators. But you need to be careful with — (double hyphen) because — is only a comment designator if it is followed by a space. Use more than 2 hyphens, or omit the space between the hyphens and the comment text, and all of the text will be part of your SQL.